The Nibiru Cataclysm is a supposed extinction event in the early 21st Century of Earth according to coded messages allegedly from aliens in the Zeta Reticuli system. Within realities where the Reticulans exist, they have proven to be humanoid, enamoured of modern pop culture and strangely concerned with events on Earth.
While the dire predictions of the prophet turned out to be false, they were enough to generate cataclysmic Ontons which have resulted in actual extinction events in some Branes.
Nibiru
Within the realities where the onton has become real, Nibiru is a large, migratory planetary body which lost its sun eons ago. The world itself is home to multiple humanoid races with very different biomes and cultures. The various biomes represented upon Nibiru would indicate the world was constructed from the fragments of other worlds, stitched together by unknown means. This means that lush forests can border directly upon mountains and plains with almost no transition.
Each biome is ruled by a Duke or Prince, and all give fealty to the merciless ruler of Nibiru. Their royal family have ruled across all of Nibiru for generations, consolidating their rule with advanced technology and magic and by playing each of the biome rulers against each other.
Flash, Hero of Nibiru.