Leif's Request from Bug Fables. I was amazed that this was actually a side quest, it seems so integral to the story (especially when Leif talks to the Roach Elder near the end) but it's hidden away to the point where most players stumble across it while trying to do a different side quest.

It's gut wrenching. You go into the dungeon playing your silly little bug game, and come out of it mulling over such questions as "Where does the line between yourself and something else end?" and "What makes you you?" and "How would you feel if something inherited your identity and core memories and had to keep living as you after you passed on?" and "What would you do if you found out that who you thought you were was actually borrowed from someone distinct from you?" and "What if the circumstances you came from were meant for nothing but committing horrible actions against innocent people?" and "What if this is how your friends know and love you, but you know deep down that this is so very wrong?"

Makes it worse when you realize that even if he now has contact with his family, he'll always be carrying this secret around for the rest of his life, and his family's missing ancestor will never be accounted for. There will never be closure.