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.
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.