The Gyre and The Far Realm
In some distant reach of the multiverse floats The Gyre, a graveyard of planes where the last vestiges of dead worlds are drawn into a churning cloud and ultimately obliterated. Perhaps their energies are then used for the creation of new worlds, or perhaps the whole of the multiverse will be consumed one day. But those pieces of worlds that survive long enough to reach the Gyre are homes of the most dreadful and powerful beings - often those who brought about their own apocalypses.
How many of those worlds have been destroyed by corruption from the Far Realm? Few even begin to understand the alien concepts of that place, and those with the insight can become threats themselves. To most, the aberrations birthed by contact with the Far Realm are terrifying, but some begin to see that very unnaturality as something be desired, to be shared with others. Adventurers should be careful to stare into that maddening abyss - lest they become like the monsters they fight.