Fallout has been known for its bleak humor and downbeat tone going all the way back to the original 1997 RPG - a game whose "happy" ending is still a pretty major bummer. Original lead Tim Cain says he wanted the game to end much differently with a full-on party sequence.
I don't know if I need to drop a spoiler warning for a three-decade-old game, but just in case, well... there you go. Fallout has a dark ending where you can choose to join the main villain, but even if you beat the bad guys and complete your objectives, your character returns to Vault 13 and is swiftly exiled by the Overseer, who fears that their stories of adventure out in the wasteland might inspire others to abandon the Vault.
