Just randomly came across this while YouTubing.
In the platformer, Sundered, you play as a female protagonist named Eshe who, throughout the game, is offered various power-ups that can end up corrupting her if she uses all of them (at least, that's my impression based on the two minutes of research I did on this).
The video below has the three final boss fights, depending on your path, and the three possible endings. The corruption ending is the first.
https://www.youtube.com/watch?v=cBxbYA9nMsI&
Points of note:
Eshe starts off fighting her doppleganger (who, I think is supposed to represent what's left of her humanity). The doppleganger turns into some sort of Cronenbergian horror at around 0:52 (not a sexy transformation, but I'll mention it because it's there.)
Eshe turns into a glowing purple demon at around 5:16. It's a poof transformation but it's the culmination of a mental TF that's been taking place over the course of the game (probably... again, I haven't played it).