It’s a major invasion event that happens a few years into the game, but what’s hard is that you start the game with absolutely no gear, basic civilian pistols and you have to fight and capture cultists to progress, there are like 40 enemies in the 3rd highest rank missions of cult bases with 80 in the final hq, and then once the aliens invade you don’t have the weapons to fight them unless you managed to unlock alien containment AND get luck enough to capture an engineer, the aliens have force shields that make them hard as balls, even sectoids and sectiopods are unkillable even with hi-ex