BSG
It's where you get good and bad karma throughout the game and then basically everyone gets executed for being bad at the end.
No, not everyone! Just one person. Maybe.
Here's the longer explanation of the Ionian Nebula:
The most noticeable change for most of the game is that at any time, there will be three 'Allies' ensconced at various locations. When you encounter them, they can help or hurt the fleet, depending on whether they have good or bad trauma.
Everyone has trauma. Accumulate too much of the wrong sort, and you can get booted from the game. You can manage your trauma by encountering allies, because you contribute a piece of your damage to decide their replacement's mood. However, bad trauma for you is also bad trauma for the Ally. You can get more trauma in various ways, notably by landing in Sickbay or the Brig.
When the fleet hits Distance 8, you reach the Crossraods phase. Everyone (Cylons included) faces a personal crisis, the resolution of which depends on (again) your trauma. After resolving the Crossroads, if anyone has too much wrong trauma, there's an elimination! And after that, the Cylons launch an all-out assault on Galactica, the outcome of which decides the game.
In practice, eliminations don't seem to happen very often; but the threat of them changes player behaviour. Going by the bgg experience, the Ionian Nebula makes the game notably harder for the humans - more so than the Cylon Fleet. Nonetheless, it's proven popular on bgg, and is the last module that we haven't yet tried here.