False vacuum decay is in all ways the best existential threat. It is a consummation devoutly to be wished. I'm sure I don't fully understand the cosmology of it, but Wikipedia explains it something like this:
Empty space isn't free of energy; it just has the lowest amount of energy possible. But suppose that what we know of as empty space is really only at a local minimum of energy; that it's a "false vacuum", and there's a lower global minimum, or "true vacuum". The universe is stable unless something can push it up and over the bump separating the local minimum from the global minimum. If a bubble of the universe reached a true vacuum, the bubble would spread at the speed of light.
What would happen? The laws of physics would change, probably in ways inconsistent with life, possibly inconsistent with the existence of galaxies and stars, or even with the existence of matter. And we'd never know – the change would propagate at the speed of light, which means that there's no way to detect it; it reaches you at the same time as the light from other places being destroyed. Amazing.
Wikipedia: False vacuum decay (www)
In Lord Dunsany's "Gods of Pegāna, the chief god Mana-Yood-Sushai created the gods, and fell asleep, and is kept asleep by the drumming of Skarl, one of the gods. The universe itself is dreamt by Mana-Yood-Sushai, in response to the drumming of Skarl. If Mana-Yood-Sushai should ever awaken, the universe will end. It may be that Dunsany was inspired by some accounts of the Hindu god Vishnu dreaming/imagining the universe into continued existence, but I hesitate to speak of a religion of which I am ignorant.
The motif of an oblivious god attended by musicians is echoed by H. P. Lovecraft's creation Azathoth, the mad, blind idiot daemon-sultan who writhes at the center of the universe, to the sound of drums and monotonous shrill piping. Supposing that we are dreamed into existence by this primordial horror would explain much about the world we live in.
The formless nuclear chaos of Azathoth seems a fit metaphor for the energy of the false vacuum.
Azathoth sleeps fitfully and ravening. When Azathoth awakens...
Yours for the finding of the Black Seal,
An Inhabitant of Carcosa