- 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.