Historically, the pagan faiths were gradually supplanted by Christianity and Islam, but in the game, this does not necessarily happen. Pagans may fight back in the name of the old gods, and even seek to eliminate the monotheists from Europe.
In game terms, all pagan religions fall into one of two broad groups, with a few exceptions - the offensively-oriented Germanic, Tengri and Aztec faiths, and the defensively-oriented Romuva (Baltic), Slavic, Suomenusko (Finnic), and West African. The Zunist are unique in the fact that they are something of a hybrid of the Offensive and Defensive, the Hellenic are a hybrid of Reformed and non-Reformed pagan faiths, and the generic Paganism granting no bonuses.
- 1 Early power, late weakness
- 2 Common pagan features
- 3 Flavors of paganism
- 4 Conversion
- 5 Reformation
Early power, late weakness
Playing as a Pagan offers a highly offensive, rapidly changing play-style, however stability is very low, and you have little control over your realm. It also supports early-game dominance - you are next-to-none as a Germanic ruler in 867. However, the longer the faith goes unreformed, the weaker it becomes. Pagans tend to be powerful in early years but weaken as time passes for several reasons:
- Powerful Pagan CBs allow Pagans to expand quickly. Large realms, however, are unstable due to elective gavelkind (if tribal), difficulty converting characters and provinces, higher priority for independence factions, and a large Short Reign penalty.
- The ability to raid makes some pagans uniquely wealthy in the early game. Germanic pagans do especially well since they get free ships, can navigate rivers, and are close to prime raiding targets. But later, feudal realms grow their income more quickly, while raiding becomes less feasible as targets consolidate. Feudal rulers bordering rivers will eventually increase their fort level to the point that Germanic pagans can no longer navigate those rivers.
- Defensive attrition is a powerful deterrent in the early game. However, it only applies to the homeland, and pagans have difficulty converting provinces due to low moral authority. Defensive attrition is completely negated once attackers reach Military Organization 4.
- Pagans find it difficult to switch from tribalism unless they start as feudal (Balkan states and Zunists). Tribal holdings can be upgraded quickly, but fully settled provinces can be upgraded more. Medium-sized tribal realms can muster unusually large armies by having a large demesne, and by calling vassals to war rather than raising levies. Large tribal realms do not benefit as much since vassals of vassals cannot be called.
- Pagan territories are usually significantly technologically behind all other culture's territories, with the exception of the Germanic having a tremendous lead in shipbuilding at the start of 867, as well as receiving said bonus after some time at the 769 start.
- Foreign missionaries become more effective as tribal rulers decide they are ready to feudalize.
There is hope, however: should the Pagan faiths become reformed, the faith's survival is almost set in stone. You trade some of your offensive/defensive capability for stability, faster conversion, and Great Holy Wars to combat Crusades and Jihads.
Common pagan features
Most pagans may raid neighboring provinces (and coastal provinces, if ships are available) for loot and prestige, even without a tribal government (which majority of the pagans are). They also have access to two special casus belli.
The powerful subjugation CB is usable on other pagans either once per ruler's lifetime or, if the ruler has the Become king ambition, unlimited use of subjugations within the target kingdom until the kingdom is created. Subjugation wars conquer or vassalize all territory within a de jure kingdom.
The County Conquest CB is usable on any neighboring province not of the same religion, and carries with it a truce lasting only 5 years. Germanic pagans can additionally use this on any non-pagan coastal province after the start of Viking Age in 790s. Note that this does not apply to non-Germanic pagans if they are reformed Germanic.
Most pagans have "defensive attrition", massively reducing the supply limit for non-pagans invading their homeland. This attrition only applies in provinces of the same religion as their ruler, so Germanic pagans won't lower their enemies' supply limits in Slavic territory. This severely slows down early-game conquest of the typically smaller, fractious pagan realms by expanding Abrahamics. However, invaders can negate the attrition by quickly building a fort in each province they enter, or by having Military Organization 4 in their capital. Tengri and Aztec pagans do not have defensive attrition (though Tengri invaders can ignore it).
"Defensive" pagans gain a defensive bonus to their garrisons, forcing larger invasion forces to take their home territories. "Offensive" pagans get a bonus to their levy size (+30% levy size for Germanic, for example).
Pagans have great difficulty holding large realms together. The short reign relations modifier is three times more severe for pagans. With tribal mechanics, pagans cannot raise Tribal Organization very high without having severe relations penalty for tribal Unreformed Pagan vassals (-30 for Absolute). They also cannot reform to Feudalism or a Merchant Republic unless they Reform or convert to a non-pagan religion. Pagan vassals are more likely to seek independence, and gain warscore more rapidly than others in independence wars.
Default succession law for tribal pagans is elective gavelkind. However, you may convert to organized religion and change succession law to more useful, regular gavelkind and it will remain as so even if you go back to pagan religion later. Those Pagans who start as feudal, use gavelkind though they can switch to elective gavelkind too. This means that succession crises are significantly more likely without foreplanning.
There are some exceptions, though. Zunists and Mongols may use Ultimogeniture, Feudal rulers of the Celtic culture group have the option for Tanistry, pagan merchant republics use the standard republic succession laws and nomadic pagans use normal nomadic succession.
Pagans can grant themselves divorces at will, but with a significant cash payment for the divorce and a -30 relation penalty to the divorcee's family. Male pagan rulers may take up to three concubines in addition to a wife. Having young concubines (below the age of 45) gives a monthly prestige gain, which is especially useful for tribal and nomadic pagans.
Pagans are able to hold religious feasts (except Tengri/West African). But, aside from Germanic/Zun, they are identical, except forslightly different flavor text and Slavic "Jarilo" event. However, all the random events based on traits (which drop relations by 15 for those without and raise by 15 for those with the trait) are the same, along with the end bonus of 100 piety and prestige and +15 to vassal relations for one year. Each one can only occur at a certain, and different, time each year.
All pagan faiths permit female temple holders regardless of gender law, except where noted below.
Flavors of paganism
The love of battle among these pagans means that rulers incur no relation penalty with their vassals for having troops raised. All Offensive pagans also gain a +30% bonus to their levy sizes, making up for likely lower technology and smaller fort holdings. Combined with not paying any penalty for raising their vassals' levies, this means offensive pagans can muster forces far beyond what their technological levels say they should.
The other side of the coin is that peace is seen as unbecoming. Rulers who are neither at war, nor raiding, nor bound by an active truce lose a significant amount of prestige per month. Because raiding counts as war, and they have county conquest casus belli that require no reason for war, however, few rulers need find themselves short on wars for long. If that was the case simply raising levies and toggling looter and waiting will remove the prestige penalty.
Germanic faith (more commonly known as Norse) is the faith of Scandinavian & Saxon people, worshiping deities like Odin/Wotan. Faith is widespread in earlier starting dates, among the multitude of petty rulers but is in decline from 1066 onwards. The last playable Germanic ruler available is Arnfast of Västerbotten, who dies in 1309.
While three of the faith's holy sites are in Scandinavia, the other two are in mainland Europe and often held by strong Christian rulers. Keeping moral authority high is also a problem due to weak realms who often lose religious wars.
Due to the Germanic faith's long and combative relationship with Catholic western Europe, there are a number of unique events between them. In particular, if Braunschweig is held by a Germanic ruler and a Christian ruler occupies its temple of Paderborn (which normally occurs in the Charlemagne DLC's scripted invasion of Saxony by Karl of West Francia), all Catholic and Germanic rulers will receive a pop-up that the Catholics have burned the holy site of Irminsul, causing the Germanic faith to lose 10% moral authority for 10 years.
- Coastal conquest
- Germanic faith may use the county conquest CB against any coastal province belonging to anyone not of their own faith -- Muslim, Christian or even other non-Germanic pagans are fair target -- no matter how distant.
- Great Blot
- Germanic pagans have a feast where they can sacrifice any prisoners taken to their gods. (This can be a fun event for some, especially if you just invaded France and now have 73 Catholic bishops in your dungeon, or just put down your 7th catholic revolt and have a half-dozen heresiarchs in your dungeons.) This is the only feast where piety and prestige gain are not set, and with sufficient sacrifices, can be quite significant. It begins like any feast, but after it begins, there are successive events for prisoners in your dungeons. There are 7 kinds of flavor text: 1 for Muslims, 1 for Christians, and five for Pagans. For each Muslim or Christian sacrificed, you gain 50 prestige and 25 piety, and pagans give you 25 prestige and 10 piety. You may spare a prisoner for a cost of 25 piety if you so choose. There is a limit to 4 prisoners per blot. After the sacrifices, flavor events are the same as a standard feast. Blots have an additional "cooldown time" of 9 years. It is possible to use Blot to earn the Holy Smoke achievement.
- Prepared invasion
- Germanic rulers may announce invasion on specified target and call adventurers from all over the Norse world to serve in their expedition. (Functionally creating an event army stack that grows every few months until the invasion is launched.) Prepared invasions can also be called by vassals. However, if the game starts at 769, they must wait for a special event to grant Shipbuilding II before they can rain havoc on Europe.
- Raise Runestone
- Runestones are available for Germanic religion or North Germanic culture. Raising a runestone costs 100 gold and will give you a global +10 relation bonus for two years. Upon declaring the intent to raise a runestone, you have the choice of either commemorating a parent or your own accomplishments.
- Dedicating a runestone to a parent gives different amounts of prestige depending upon the method of death. (300 for death in battle, 250 for death in a duel, 100 for death by suicide or murder by the child raising the runestone, and 200 for any other method of death.)
- Dedicating a runestone in your own memory brings up options based upon your own traits, and flavor text to match. Raising a runestone based on specific traits gives more prestige than others. (Brave, strong, proud, or genius traits give 300 prestige. Lunatic, possessed, or imbecile give 100 prestige. All other traits give 200 prestige, but have different flavor text.)
- Eventually an event triggers (often in the 1100s) that informs you that paper has replaced runestones, disabling creation for the rest of the game.
- River travel
- Germanic pagans are masters of coastal and river warfare. They may sail up navigable rivers, and even cross overland portages between major rivers. (This allows them to functionally travel directly through Continental Europe by boat into the Caspian or Black Seas, and through there to the Mediterranean.)
- Garrison/Troop bonus
- On top of 30% levy increase, Unreformed Germanic also gain a +50% retinue supply bonus, but lose this bonus upon reformation. Any established retinues remain and can be reinforced until dismissed.
- Unique holy order creation conditions
- The Jomsvikings are unique among pagan holy orders in that they may form regardless of reformation. The conditions are as follows:
- The year is at least 920
- Either Stettin, Wolgast, Rügen or Werle has a free holding
- The count holding the chosen county is Germanic or Reformed Germanic, and accepts the creation
Holy sites: Zeeland, Braunschweig, Uppland, Sjaelland, and Naumadal.
Tengri, worship of the Sky Father, is the religion of the Steppe and the bulk of its followers are nomads, whose lands stretch from Danube to Mongolia. While its holy sites are in locations which are easily taken, most of them lack a temple holding which means holy sites only give half of the usual Moral Authority, forcing Tengri ruler to either build temples or take control of all holy sites. However, Tengri faith does not die out during the entire period of the game and there are always relatively strong rulers for it in the game.
- Ancestor Worship
- This is the same religious decision that defensive pagans have.
- Sky Burial
- Nomadic Tengri may perform Sky Burial.
- Tribal invasion
- Non-nomadic Tengri pagans have access to the incredibly powerful tribal invasion CB, allowing for unlimited wars for entire de jure kingdoms at a time
- Garrison/Troop bonus
- Tengri religion gives +30% defensive/offensive power to light cavalry units on top of the Levy bonus. Tribal rulers who are Tengri (or Altaic culture) also have a slightly different troop composition for ”Raise Tribal Army” decision.
- Tengri paganism does not permit female temple holders.
Holy Sites: Itil, Tobol, Crimea, Syr Darya, and Otuken. Note that many of these lack temple holdings in early starts.
Aztec religion only appears in-game if Sunset Invasion DLC is in use. Aztec religion has no special mechanics, having the usual set of Offensive Pagan mechanics.
Reforming the Aztec faith is possible but extremely difficult. Acquiring the faith to begin with requires you to convert religions or use Ruler Designer, and its holy sites are split up between Italy, England, Spain, France, and West Africa.
- Human Sacrifice
- Aztec pagans can sacrifice non-Aztec characters from controlled provinces to the gods. It is possible to use Aztec human sacrifice to gain the Holy Smoke achievement.
- Tribal invasion
- Aztec pagans gain access to the Tribal Invasion CB, allowing unlimited wars for entire de jure kingdoms at a time.
- Garrison/Troop bonus
- Aztec pagans gain 30% to levy size.
- Aztec paganism does not permit female temple holders.
The principal ability of defensive pagans (Romuva, Slavic, Suomenusko & West African) is a tremendous boost in combat when fighting on home ground. All units gain 80% to defense when fighting enemies in a province of the combatant's religion. This applies even when one is the attacker, and on territory held by another lord; Only the dominant religion of the province you are standing upon matters. This means you may invade territory held by another faith, but still gain the defensive bonus so long as the battle takes place upon land of your faith. (Such as reconquering Slavic lands as a Slav from Christians.) This allows these pagans to beat enemy armies they are half the size of with ease.
Defensive pagan holdings also have larger garrisons (bonus varies per religion), making them harder to siege. This is in addition to the pagan homeland attrition penalty, which forces the large numbers of troops needed to invade a defensive pagan home territory to face severe attrition for the duration of any siege. Such penalties usually function to prevent any rival faith from taking their lands in spite of being relatively weak and of a rival faith that normally could be easily holy warred.
The pagan homeland attrition penalty does not function, however, against other unreformed pagans. This allows other pagans (especially Tengri and Germanic pagans) to fairly easily invade other pagans with conquest and subjugation casus belli. The homeland attrition penalty does apply to reformed pagans of another faith, but the "old faith" heresies have no such protection against the reformed churches of their specific branch of paganism.
All defensive pagans have access to Ancestor Worship & Choose Patron deity decision. Additionally, those defensive pagans who are tribal, may call forth Devout Warriors.
- Ancestor worship
- Ancestor worship is a decision that can be taken every 10 years. In it, you ask your ancestors for various favors and offer them sacrifices, either money or human, to make sure they answer your calls. The outcome of sacrifice is always somewhat random but various decisions you do increase your rulers piety nonetheless.
- Patron deities
- Defensive pagans may choose a patron deity. This gives +2 to one ability and -1 to another. In addition to this, several events dedicated to said deities can trigger.
- Summon Devout Warriors
- This decision is available for defensive tribal pagans, whether religion is reformed or not. If you are attacked by ruler of different faith, you can spend 200 piety to summon up a group of warriors to defend your realm, much like with Raise Tribal Army decision.
Romuva is the faith of the peoples on the east coast of the Baltic Sea, worshiping such deities as Dievas, Perkunas, and Laima. Three of the religion's holy sites are located within de-jure Lithuania, making it perhaps the easiest pagan religion to reform. Romuva does not disappear from the world at any starting date but will eventually get pushed back by Christians. After the 1200's, most of Lithuania is feudal and by 1337, it stands as last pagan kingdom in Europe, surrounded by Teutonic Order, Golden Horde, Poland & Novgorod.
- Resistance to conversion
- AI Romuva rulers and Romuva provinces have increased resistance to conversion from organized religions.
- Romuva patron deities
- Dievas (+2 diplomacy, -1 intrigue), Velnias (+2 intrigue, -1 diplomacy), Perkunas (+2 martial, -1 learning), Zemyna (+2 stewardship, -1 martial)
- Uzgavenes festival
- This functions as standard pagan feast, granting ruler 100 piety & prestige.
- Garrison/Troop bonus
- Unreformed Romuva pagans get +10% to levy size and +30% to garrison size. Both bonuses change to +20% after reformation.
Holy sites: Rugen (shared with Slavic), Chelminskie, Podlasie, Zemigalians, and Bryansk.
The Slavic faith is followed, as the name suggests, by the Slavic peoples of Europe. The faith is widespread in early starting dates, and some of the realms, such as Croatia, are already feudal rather than tribal. However, the religion's holy sites are scattered far and wide making reformation very hard. The Slavic faith is almost gone in the 1066 starting date, and the faith disappears from the world entirely by 1200. The last playable Slavic ruler is chief Testislaw of Dyrmin.
- Jarilo festival
- This is like other pagan feasts, but one of the guests (chosen at random) becomes a "Jarilo" which improves said vassal's relations with you by 25.
- Slavic patron deities
- Perun (+2 martial, -1 intrigue), Veles (+2 intrigue, martial -1), Jarilo (+2 stewardship, -1 martial), Morana (+2 learning, -1 stewardship)
- Garrison/Troop bonus
- Unreformed Slavic pagans get +10% to levy size and +30% to garrison size. Both bonuses change to +20% after reformation.
Holy sites: Novgorod (shared with Suomenusko), Kiev, Plock, Birlad, and Rugen (shared with Romuva).
Start in Novgorod, Poland, or Kiev and form your corresponding de jure kingdom first, then work your way to the other two holy sites.
- Ilmen in 769 already holds Novgorod as its capital, is a decently strong start, and has the advantage of being able to flip to Russian culture once the faith is reformed, allowing the acquisition of the "Russkaya Pravda" achievement. Ilmen is also surrounded primarily by weak Suomenusko and Romuva pagans and is shielded by an ocean from the Norse until the Viking Age event triggers, giving plenty of time to solidify power.
- Kiev in 769 is notably a start whose default ruler, Dobrava Turov, is a woman. Playing from this start is more difficult, as Kiev is bordered to the south by the Tengri Khaganate of Magyars.
- Rurik Rurikid, King of Könugarðr in 867, is the historical founder of the Russian Empire. He is a culturally Norse Germanic pagan but has a Slavic son. You can use the Germanic faith's powerful CBs and shipbuilding buffs for expansion and raiding, then be succeeded by a Slavic ruler and reform.
Suomenusko, religion of the Finno-Ugric people stretches from the Baltic Sea to Siberia. Its holy sites are scattered around the map but are mostly controlled by right religion, even in 1066 starting date. The easiest route to reform the faith is with a Finnish ruler as de-jure Finland has two holy sites. The nearby county of Novgorod has a third holy site, which can be conquered relatively easily. Bear in mind that in 1066 Novgorod is Christian and Rurikid dynasty which holds it, has multiple potential allies from other dynasty members.
Suomenusko retains a sizable number of provinces throughout the game and it is very easy to put the fear of Ukko to other religions, whether you start in 769, 867 or 1066. However, by 1337 only two chiefs in northern Finland/Sweden follow the faith, with the rest of Finno-Ugric lands being divided between Catholic/Orthodox Christians and the Sunni Golden Horde.
- This special event relating to bear hunt can trigger and it offers a ruler a way to spend money for prestige or gain gold by losing prestige (if greedy )
- Suomenusko patron deities
- Ukko (+2 stewardship, -1 intrigue), Akka (+2 diplomacy, -1 martial), Tapio & Mielikki (+2 martial, -1 learning), Lempo (+2 intrigue, -1 martial)
- Ukon Juhla
- This functions as a standard pagan feast, granting ruler 100 piety & prestige.
- Garrison/Troop bonus
- Unreformed Suomenusko pagans get +40% to garrison size. Reformed Suomenusko pagans get +10% to levy size and +30% to garrisons.
Holy sites: Perm, Ryazan, Novgorod (shared with Slavic), Osel, and Kexholm.
West African 
The West African faith is followed mainly in area of Mali, where two of its holy sites are located. The faith is in a strong position in the 769 start as West Africa is ruled by tiny Muslim realms which can easily be conquered. In later starts, expanding north becomes increasingly difficult and by 1066, the rulers of the area have largely converted to Islam. However, there are always playable rulers and provinces for this particular pagan faith in West Africa.
- No short reign penalty
- Unreformed West African faith does not suffer from extra "short reign" penalty like other non-Zunist pagans.
- West African patron deities
- Yemoja (+2 diplomacy, -1 intrigue), Ekwensu (+2 stewardship, -1 learning), Orunmila (+2 learning. -1 intrigue), Shango (+2 martial, -1 diplomacy)
- Garrison/Troop bonus
- West African pagans, both unreformed & reformed, receive +40% to garrison size.
Holy sites: Timbuktu, Mali, Marrakech, Atlas Mnt (shared with Ibadi Islam), and Medjerda.
Faith of Zun is a sun-worshiping pagan religion introduced in the Charlemagne expansion. The only realm following this faith is Zunbil Satrapy in western Afghanistan. They are only playable in 769 bookmark unless Ruler Designer is used. By 867, the entire dynasty has converted to Hindu/Sunni faith and not even a single courtier of this religion remains in the world. However, the Sons of Abraham demon child event chain will create Zunist "witches", who can be married matrilineally or assigned as guardians to children with the Faith or Heritage focus.
Zunist mechanics are a hybrid of defensive/offensive pagans and organized religions. They start as feudal, are unable to raid, have lesser chance to convert county religion, do not suffer from increased "short reign" penalties and may use ultimogeniture succession law instead of gavelkind. Zun can also be reformed like other pagan faiths. This grants Zunist rulers a holy war Casus Belli, religious head (Church of Zun), a holy order (Knights of the Sun) and other perks of a proper organized religion. In Monks and Mystics Zunists are also unique among pagans in that their rulers may join the Hermetic Society.
- Dharmic/Mazdan intermarriage
- Unlike other pagan faiths, Zunists are able to intermarry with non-nomadic Mazdan/Dharmic rulers (and vice versa) so they are not entirely without allies. Unfortunately, the strongest of these are Hindu, who will reject marriage arrangements due to your lack of caste. However, Hindu rulers will allow you to take concubines, who you can then decide to marry. Another solution is to convert to Hinduism so you can acquire the Kshatriya caste, then convert back to Zunism.
- Festival of the Sun
- Like other pagan feasts, this gives ruler piety and prestige. However, it also has event "Walking under the Sun" which has a chance of giving traits to your ruler, including physical ones like Strong .
- Judged by Zun
- In addition to usual choices of execute/ransom/release prisoner, Zunist can leave prisoners to a desert to be "judged by the sun". This usually kills said prisoner but few other outcomes are also possible.
- Garrison/Troop bonus
- Zunists receive increased levies (+30%) & defensive attrition in Zunist provinces and better combat power (+20% defensive/offensive) for heavy infantry units. Unlike defensive pagans, they do not receive 80% defensive bonus in home provinces.
- Empire of the Sun
- This achievement is earned by ruling an empire-level title as reformed Zun ruler in an Ironman game.
- Zunism does not permit female temple holders.
Holy sites: Baghdad, Bost, Cairo, Kabul, and Multan. Baghdad and Cairo are distant and usually controlled by major Muslim powers. The other three are all in or near Afghanistan, making reformation possible.
- Tips & Strategies
- Changing culture: Raiding temples can help you gain the moral authority needed for reformation with only the three nearby holy sites. Unlike other Pagans, feudal Zunists are unable to raid but there are several cultures in game which allow this. Altaic culture group is a obvious choice due to number of nomadic realms north of Afghanistan. Altaic group also allows Tribal Invasions if independent. Norse is even better, because its cultural buildings and cultural retinues benefit from the Zun faith's +20% offense/defense to heavy infantry. The Varangian Guard, if the Byzantine Emperor forms it, can supply Norse educators.
- Serving the Caliph: Swearing fealty leaves you relatively free to pursue expansion, while your liege deters attackers and crushes revolts. It also lets you claim the holy site of Baghdad. It is recommended to have a kingdom-level title, as your Iqta liege can freely revoke duchies. Note that it is possible to reform the faith even while being a vassal, although you'll only get half the moral authority from each holy site. This strategy is easier with patch 2.7, as you can falsely convert to Sunni Islam to shield you from religious revocation while developing a network of like-minded Secretly Zun Pagan rulers.
There are two special pagan faiths that normally only appear in-game as the listed religions of long-dead historical figures. Neither is normally playable (not even with Ruler Designer), unless you use Mods to make them playable or alter the character database, or exploit loopholes.
Hellenism, the faith of the Greco-Roman world before the rise of Christianity, is in game play terms a hybrid, combining some features of organized religions with some features of unreformed pagans.
- Organized religion
- Hellenism gets the base 20% moral authority, unrestricted succession and crown authority laws, lack of defensive attrition, absence of short reign penalties, and ability to demand religious conversion that organized religions do. It cannot be reformed as it is already an organized religion.
- Slowed conversion speed
- The Court Chaplain's Proselytize mission is less effective.
- Casus Belli
- Hellenic pagans have access to the county conquest and pagan subjugation CBs, but not the holy war CB.
Hellenism may become playable by exploiting certain events that occur only for immortals, in which an immortal horse attacks the player. If the player can diplomatically resolve the situation, the horse can be granted titles or used to educate children, allowing Hellenism to be played. Though Hellenism has no holy order, Hellenic rulers are permitted to join the Hermetic Society in Monks and Mystics.
Holy sites: Abydos, Thessalonica, Athens, Rome, and Alexandria.
Generic Pagan 
A generic Pagan faith is used in the history files to represent everyone from pre-Christian Irish kings to pre-Islamic Arabs. This faith is exceptionally limited - it has all the disadvantages of unreformed paganism and none of the benefits, nor can it be reformed. Generic Pagan rulers cannot be created with Ruler Designer but there are few events in game which spawn Pagan courtiers to game, which permit granting of titles or education of children.
Holy sites: Rome, Västergötland, Mecca, Jerusalem, and Ghana.
The lack of a formal religious tradition means that pagans have a harder time spreading their faith outside pagan lands. Seers (court chaplains) have a significant penalty to the chance to convert provinces on top of their likely-lower moral authority, and vassals cannot be demanded to convert.
Inversely, it is much easier for organized religions to convert pagans. Foreign rulers will sometimes send missionaries. If allowed to spread their religion, they will in time ask the ruler to convert. Pagans being attacked in a holy war by Christians or Muslims may choose to save their realms and end the war by accepting the faith of the invaders. Lastly, pagans may, by decision, convert to the religion of a spouse or concubine of a different faith.
Reforming a pagan faith establishes a formal religious hierarchy and scripture - adapting elements of the Abrahamic faiths in order to more effectively oppose them. In game terms, reformed pagans no longer face the restrictions on crown authority, succession, independence wars and conversion that other pagans do.
The religion now has a formal head, who gives the faith a permanent +20% to moral authority, and who may, after the Crusades and Jihads are unlocked, call Great Holy Wars. The Germanic reformer becomes the Fylkir, a ducal-level secular leader similar to an Islamic Caliph. The other pagan faiths have a duke-level religious leader who becomes a vassal of the reforming ruler, similar to the Orthodox religion's patriarch. With the exception of the reformed Germanic religion, the faith will also immediately gain a holy order.
When the religion is reformed, rulers of the faith may choose to accept or reject the new order of things. Those who stick with the old ways are considered heretics by the reformed faith, and may be targeted by holy wars.
Reforming a faith costs 750 piety. It requires either control of three holy sites and 50% moral authority, or else control of all five holy sites. If the last two holy sites are difficult to reach, the best way to obtain the necessary moral authority is through county conquests and raids. Each temple looted in a raid gives +1% to moral authority.
Note that Germanic pagans have as a holy site the county of Braunschweig, and taking that county will trigger the Catholic crusades (as well as the formation of every Catholic Holy Order but the Knights Templar) at the time that you take Braunschweig, so long as the year is 900 and the province is Christian. If you need Braunschweig to reform the Germanic faith it is best to conquer it in 769, while the Saxons still control it.
Reformed pagans will gain a sudden bonus to their conversion rates. In fact, if they have high moral authority, and have damaged rival religions' moral authority with their conquests and stolen holy sites, or control unreformed pagan lands, they will soon be flooded with notices of conversions of territories that are held by their vassals, who will put forth serious effort to convert their lands to their new faith.
Reformed religions gain access to holy wars, usable on any target of a different faith (including other pagans). They also gain access to Great Holy Wars if the Catholics have unlocked Crusades or the Muslims have unlocked Jihads. GHWs are especially useful for Germanic Fylkirs, who can select the target and timing of the attack.
To help in the struggle against unbelievers, each reformed faith gets a single holy order. (There is one exception. Germanic pagans do not have to reform to create the Jomsvikings; instead, they must control Stettin, Wolgast, Rügen or Werle.)
For tribal pagans, reforming makes it easier to raise tribal organization. Tribal vassals who follow in reformation will no longer have opinion penalties for higher levels of tribal organization, no longer faction to lower tribal organization, and will usually vote in favor of increasing tribal organization.
Reformed offensive pagans no longer lose prestige when at peace.
The short reign penalty is reduced to merely double the normal amount for Germanic pagans among the same Germanic faith, but Old Germanic pagans will have the full -45 penalty in addition to the heretic penalty. Other pagan faiths have different modifiers.
Reformed pagans no longer get a defense attrition bonus, and will incur attrition in the territory of unreformed pagans, except those who follow the old form of their own religion. Germanic rulers lose their retinue size bonus and now get penalties for raised levies like other rulers.
Reformed religions lose access to subjugation wars, one of the fastest ways for pagans to expand. They keep access to county conquests and raids.
Reforming the religion will cause a schism in the pagan faith - the reformer will be of the new religion, and so will their direct vassals, but others have a random chance (or a choice if the player) of being one faith or the other. Old religion followers will be heretics with a -35 penalty to relations to members of the reformed faith. If a king of a faith stays true to the old ways, and many vassals convert to the new religion, this can cause tremendous upheaval in a nation, and lead to multiple simultaneous revolts. Expect frequent raids and wars between realms that accept the new faith and those that reject it.
Reformation tends to grant benefits of stability and the capacity to declare wars on larger-scale targets that smaller pagan nations couldn't attack, but lose many of the benefits that would help a small pagan nation expand more readily or defend itself from larger targets. As such, it is best used when the player is ready for their nation to "mature" from aggressive expansion against small targets to needing to maintain an empire.
Because of this, it is usually strategically advisable to hold off on reforming your religion until your realm is at a multiple-kingdom level, and to make sure to exploit any subjugation or county conquest CBs against nearby easy target pagans before reforming your religion.
Now that you have no fast way to expand through pagan lands, you may wish to convert to a culture that allows use of the Tribal Invasion CB, so you can quickly expand through infidel lands instead.