- A new invading threat arrives at a random point in the mid to late game, conquering from the West
- Unique new portraits, on-map shields and units for the great Mesoamerican Menace
- Cower in fear at the Aztec gods: New Aztec culture and religion
- Human Sacrifice! When a province is lost, no-one is safe from the threat of losing their hearts on the obsidian altars
- Survive the terrible disease brought by the pagan oppressors: A new plague from across the Ocean
There was some criticism of the expansion for its alternate history setting, their games being known for attention to historical detail. The CKII project lead explained that its development was mostly outsourced or done on their spare time, and that this DLC should simply be considered as optional content. .
- Aztec invasion idea came from a symmetry of the Mongol horde invasion, to offset the imbalance of Western edge of the map being very safe compared to the Eastern edge.
- If you have both The Old Gods and Sunset Invasion enabled, and north_germanic culture, an event around year 1010 describes how the "skrælings" of Vinland captured several Viking longships. This may serve as an explanation for the Aztec's ahistoric rise in the expansion's alternate timeline.
- If we ignore the fact that Aztecs did not launch any invasion in the Old World in real life nor had the shipbuilding technology to do so, the Aztec Empire simply didn't exist during the 13th century by the time of their ingame invasion, their capital Tenochtitlan having been founded in the early 14th century. Also, Aztecs lived in modern-day Mexico, thousands of kilometers from the supposed location of Vinland (estimated to be modern-day Newfoundland in Canada), an area far from any Aztec influence by the time of the height of their empire. The logistics of sending a hundred thousands warriors over the Atlantic Ocean is also considerable, invasions of this scale over such distances would not be possible until the modern era. Vikings having sailed by Iceland and Greenland to reach Vinland, this means Aztecs have surpassed their seefaring abilities, to match that of 15th century Europe's and China's. This means that in Sunset Invasion's timeline, the Aztec Empire formed centuries earlier and is much more powerful and advanced than in our timeline, probably being the most advanced realm in the world.
- Although the Aztec disease is a parallel to real-life smallpox who killed most of Native Americans, Americas were relatively disease-free when compared to Eurasia, making another difference from our timeline.
- It is however curious that the Aztecs launched such an impressive invasion without any form of reconnaissance that surely wouldn't have gone unnoticed. Considering the vast lands available in North America, invading the other side of Atlantic Ocean would hardly make sense unless North America is already entirely settled. Presumably, the explorers mentioned in the Realm of the Skrælings event revealed some knowledge of the Old World to their captors.