Yeah, you read that headline right. I was just as confused about the announcement of Sunset Invasion , the latest DLC for Crusader Kings II releasing on November 15, as you were...at first. To quote from the official release: "In the latest DLC for the critically-acclaimed strategy/RPG Crusader Kings II, Paradox Interactive and Paradox Development Studios pose the question—what would have happened if the Aztecs invaded 13th Century Europe?" What it means is that the Count of Anjou could have his heart ripped out of his chest and sacrificed to Quetzalcoatl. I think that's pretty awesome.
CKII's previous expansions, Sword of Islam and Legacy of Rome, have been focused on fleshing out existing, historical civilizations. Sunset Invasion expands the game's already rich palate for alternate history by turning the tables in a scenario that explores the ramifications of the Mesoamericans bringing the fight to Europe, rather than the other way around. The Aztecs will show up around the middle of the 400-odd-year timeframe covered by Crusader Kings, arriving at a random location on the Western coast. The irony of the fact that this will likely be Spain in many games, I don't think is lost on anyone.
The Aztecs will bring with them a new culture, a new religion, and some kind of terrible plague (again, no irony is lost) to shake up the European balance of power in violent, human-sacrifice-laden ways. Unfortunately, you won't be able to play as the Aztecs. The invasion is only two weeks out, so start reinforcing those castle walls and sharpening your swords. You're going to need them.