Reminds me of the conclusion of one of the longest campaigns I ever ran. It was AD&D. The Cliff Notes version is that the elven wizard chose in the moment to save his human knight friend from imminent death at the hands of one of the arch villains rather than stop the epic, world-shaking ritual of ultimate doom. Defiant to the end, even though the ritual had been completed, as the ultimately powerful energies were in the process of being released, the wizard decided to break the arcane glyph circle (which was peppered with numerous artifacts it was actively draining of their powers). So I blew up the world. End of game. It was an awesome finale.
Years later, we decided to play Dark Sun. Same players. I created a campaign around the idea of a mysterious and powerful, ancient elf preserver living in an isolated green oasis. The PCs end up working for this strange, scarred old magic-user who mumbles about "the time before" or some such. Their job eventually became to scouring the bleak, broken desertscape for ancient, arcane knickknacks the old man needed to complete some obscure, powerful spell. That was the bulk of the campaign's theme.
It wasn't until the needed items were all gathered for the ritual, and we were nearing the end of this campaign, that the wizard explained the PCs' final task. The time travel spell would send them back to many centuries earlier, before the breaking of the world. Their mission was to stop the young version of him from screwing up by allowing a powerful ritual to be completed which would destroyed the world as he knew it.
Needless to say, the looks of the players' faces, when they realized their new characters had to go back and stop their old characters from the last campaign ending the world, was truly priceless. One of my finest 'big reveals' in my decades long DMing career. And especially the player of the elf wizard who realized his old PC was their new benefactor all along. Good stuff.