Concept: a D&D campaign where the players start out by creating high-level characters and jump straight into a final boss fight with an evil chronomancer. Whether they win or lose, they’re just barely too late to prevent an apocalyptic ritual that unravels the timestream.
Once the prologue is complete, they level their characters down by one level and play out the adventure immediately preceding the fatal confrontation. The next session, they level down again and play out the adventure before that, and so forth, with each adventure successively further back. No separation between player and character knowledge is enforced, effectively affording the player characters knowledge of future events (i.e., the “previous” adventures they’ve already played); exploiting this knowledge is encouraged.
Armed this this information, the player characters are able to make changes to history. They can’t prevent their final confrontation with the chronomancer without causing all sorts of nasty paradoxes, but due to the vortex of Time Fuckery™ surrounding it, the battle itself is exempt – i.e., they’re allowed change history in ways that give themselves advantages in the fight. Changes “close” to the event (i.e., only a couple of sessions in) confer only modest advantages, while changes further back in history (i.e., many sessions into the campaign) are more influential.
Every adventure ends with a flash-forward to the final battle with circumstances revised to reflect whatever changes to history the player characters just made. The campaign ends when they successfully prevent the chronomancer from blowing up the space-time continuum, thereby locking in the new timeline.
(If they manage to get all the way to level one without stopping the chronomancer, their “first” adventure has them bumping into a random NPC who they now realise is a low level, not-yet-evil version of that same chronomancer. The same no-preventing-the-final-battle rule applies, so they can’t just kill the poor schmuck, but now they’ve got an opportunity to really throw a wrench into the works!)
