I have a PC who knows a fair amount about prophecies/beliefs concerning "the end of the multiverse." In the Planescape cosmology, with its numerous pantheons, there are probably a lot of those. I know a bit about Ragnarok for the Norse pantheon in PS, and the gods of Oerth may have something similar relating to the return of Tharizdun.
What other prophecies/beliefs of this type exist in Planescape? I figure some would probably exist even if they haven't previously been stated in canon. (For example, Shar. Given that she wants to return the universe to its original state of non-being, there are probably some prophecies of this type associated with her worshippers.)
There's the prophecy (The Sargonne Prophecies) that the illithids will one day form a vast empire dominating the dying multiverse in the unspeakably far future.
There's the prophecy that the General of Gehenna will move its Crawling Citadel and bind the other fiendish races under its command, moving to annihilate the forces of good.
There's the prophecy that Asmodeus will win the Blood War, break free of the cruel mathematics of his plane, and rewrite the laws of all existence in his image.
There may be counter-prophecies, too: that the fiends will wipe each other out and the celestials will sweep across the planes to bind everything with their unforgiving chains of virtue, exterminating those who fail to measure up to their ideals.
There's the Dustman prophecy that one day all will comprehend the lie of life and True Death will fall upon all, eternal peace without the struggles of false life.
There's the Doomguard prophecy that one day Entropy will conquer all, and all liquids will dry to salt, then everything flammable will burn to ash, then everything solid will crumble to dust, then all gas and dust will dissolve into vacuum.
The Godsman prophecy that all mortals will ascend into divinity, and then beyond divinity, leaving the multiverse behind like moths leaving their cocoon.
There's the prophecy that that one day the wards of Bolothamogg will falter and the Far Realm will pour into the multiverse, warping everything with its infinite madness.
There's the prophecy that the Lady of Pain will die by the hands of Orcus (or Vecna, or whoever) and Sigil will fall from the Spire.
One day Ra will die by the hands of Apep and all will plunge into eternal night.
One day Zeus will be killed by his youngest son, his brothers and sisters will be locked away in Tarterus, and a new generation of gods will rule from Olympus.
One day the Central American sun-god Tonatiuh will fail to receive enough sacrifices to continue across the sky and the age of the Fifth Sun will end.
Cryonax will turn his plane into a full elemental plane, then go further and freeze all that is.
The Sleeping Ones in the Plane of Ice will awake, and the draedens will reclaim the multiverse, reducing all matter to back to the primal void.
The rilmani will conquer the Great Wheel, making all the planes of alignment into the featureless planes of the Outlands.
The slaadi will destroy the Spawning Stone and transform the limited prison-reality we know into a multiverse of infinite possibilities.
The formians will conquer all, reducing the modrons to mere drones caring for their hives and all else to slaves.