Either one or both works.
Mine is completing the Pokedex in the original Pokemon games. All you get is just Professor Oak giving you a wink and a small few second cut scene. And a congratulations text. Imagine spending all of your time then, getting all 151 and even 252 pokemon just for that? Yeah no thanks, I never completed the pokedex.
Going the Joja-Route in Stardew Valley. I say this mainly because, it is what you make of it. You forfeit being able to complete the Community Center by earning things, when you sign your soul away to Joja. What I would’ve liked is seeing Pierre go out of business completely. I just think that would add a route of depth in the game where you have to make ends meet through Joja because Pierre is permanently closed.
But, that doesn’t happen, he’ll still be in business despite his depression about Joja running things. Kindof ruins the whole concept of doing it for the achievements even.
You killed the ultimate boss; now with their drop you are the setting’s ultimate boss. You just need to wait for another plucky young upstart to rise and take you down.
Diablo spoiler?
Ehhhhh… Kinda, not really. The wizard goes mad, the rogue aligns with evil and the warrior failed to contain the big evil and is possessed by it.
Doesn’t sound like final boss to me.
I may be wrong, didn’t follow the series much, but wasn’t Diablo2 BBEG the player from the first one turned evil?
Yeah, as I said, it was the warrior, who took diablo’s soulstone with himself, but succumbed to evil and was possessed by Diablo. So, yeah, tuned evil. Still, at that point I wouldn’t call that body the player from the first game.
Afaik originally in D2 we were supposed to kill him and that would be it, but the animator company decided that it would be cool to animate some dude piercing his forehead with a stone, and since there wasn’t anymore dev time Blizzard North decided to go with it. That gave way to to justification for the corruption/possession of the D1 warrior character and thus the story of D2 and kinda D3.
Or many of the Soulsborne games.
Tap for spoiler
Replacing Gehrman in one of the Bloodborne endings being the most direct example.
Rarely even happens in-universe.