Oblivion is the common knowledge name given to a ‘place’ and type of existence in my main OU! It’s the ‘space in between spaces’, ‘the void’, and ‘nothing.’ Like the concept of negative space/energy in a way? It’s even negative energy to death, because Oblivion and to be ‘of Oblivion’ means you are neither alive or dead, neither existing or not existing. Oblivion just is.
It will slowly consume the entirety of existence (what’s left of Eternity’s corpse basically) and in a way you can consider it a form of death and decay but even Death itself fears Oblivion.
The current King of it has slowed it’s consumption of existence but it can’t be stopped, all he can do is slow it and if he were ever to fall as the ‘King’ will within Oblivion, then whoever takes his place would likely stop slowing it and let things resume at the speed it was beforehand, which would end existence quickly.

Also Oblivion is sentient and a hive mind to a degree (not completely tho), and has a multitude of different identities within in it and all those identities are beings that were consumed by it. The King (strongest identity/will) would be Celso, and who he was before becoming of Oblivion was also the first to join Oblivion (tho not really by choice). He is always in a constant battle of wills with other strong identities within Oblivion that want to usurp him. Power is incredibly important within Oblivion, and the only way to gain power is to devour energy/pieces of existence.

(I have difficulties sometimes explaining how the ‘hivemind’ works for Oblivion and how it is sentient but I’ve found a few other medias that can help explain it more. Sort of how the Markers are in Dead Space and how the Necromorphs react to it/obey it but some are a bit more individual and can act outside of the Marker’s wants, and also the video game called Prototype and how the blacklight virus interacts with it’s main host personality, Alex, and then all the other personalities/identities/memories that have been consumed by Alex over the course of the game. Those are the two best examples I can give of things that can sort of help explain (if you look them up and do some reading) Oblivion’s ‘hivemind’ more and how Celso and other strong identities/wills can be their own selves but are also Oblivion itself too.)