Elemental creature that is killed layer by layer by the party. So for example, they first meet it as a frost elemental and when they smack it to death or something else stupid it becomes a fire elemental that they have to kill and so on. This can either be one encounter or they meet it a various plot points. These elementals aren't super smart but tough.
Anyway, final form is a time entity of some sort that sends them through time to try and kill them while making them deal with the consequences of their actions in the game prior. Separately and together using things they've said about their backstory and in game play. When the time god tells them they will have to face themselves it's not a fight. It's literally they can kill the level 3 versions of themselves and continue at their current level but they will lose something important to their character (someone their character loves, prized possessions, memories, others having memories of them, fame, etc) and they must fight the time god who predicts he will kill two of them or they can let the level 3 versions of themselves kill them and restart where there name isn't tarnished and the time god loses powers and hp for each player that does so.
They don't have to choose as a group but whoever lets their 3rd level character lives becomes that with none of the memories or items after third level in the campaign (NOTE TO DMS: Maybe have a part where they're at 3rd level where someone tells them they've been missing for a few weeks but it's like the day after for them). However, if they kill their 3rd level selves they will all be sent back in time to that 3rd level timeframe to have to re-kill the elemental levels in stages that are now harder because the multi-elemental foe has learned. The time god sends their elemental selves throughout time creating paradoxes and new versions of these floor bosses.
