For anyone who's played Dragon Age: Origins, the Mages' Tower. Specifically, the Sloth Demon.
I... don't even know where to begin. Not only is it thrust on you completely out-of-the-blue, but is it also one of the longest segments of the game and it is also incredibly annoying. You're limited to only one party member; you have to use a gimmicky, jacked-up transformation system (sometimes mid-combat); you have limited ability use, and are most of the time restricted further to those only known by the form you've been forced to transform into; most of the area is a maze; ect. ect.
There are (multiple) mods for DA:O specifically designed to remove this entire segment of the game. That's how much the community doesn't like it.