The Leveler is a transient divinity that seeks to destroy the world and all life with it. The Leveller during each age of darkness would rise and try to destroy the world but would be defeated by a hero who would usher in an age of light.
But this wouldn't kill the Leveler. A twisted and ironic attribute that the Leveller has is that it would return in the next age, stronger then before and possessing the body and soul of the hero who defeated it. Hence the hero that saves the world from darkness would be doomed to destroy it and all that they had fought hard to save.
The Leveler had gone through many cycles, assuming many different identities but the known incarnation, and the hero who defeated them, were Soroganth the Flayed, Moagim (Tireces the Immortal), Mjarin, and Balor (Connacht the Wolf).
At the end of the Great War, Balor, the current incarnation and the most powerful, having nearly destroyed the known world was beheaded and, in an attempt to make sure the Leveler would never return, his head was cast into the Great Devoid. The attempt appeared successful and the Leveler was finally destroyed, but then no one could be sure until the next cycle started.