D&D 5e game:I have two planets orbiting each other.
1200 years ago it was just a moon, then somehow the moon fell and almost ended the world.
But the next day there was a moon again.
Really it's another version of the same world from a parallel timeline. The act of summoning it led the powers that be to seal the system in a bubble plane to keep the chaos from spreading.
The wizard who did it fell in love with himself from another timeline. The BBEG is two liches who are each other's phylactery. If both cannot be defeated before the first revives, there is no way to stop them.
Such a tragic story, to do that much for love and then only be vulnerable when you're near your love.
Their name is Zeitounessian, and I have a party on each world trying to stop him.
Neither party has yet determined that there are two of him.
Have fun with it!