The following details come from an official Trillion: God of Destruction press release:

More than a thousand years passed since the legendary battle against the Heavens. The demons of the Underworld lived an evil, yet fulfilling life in peace. And so it remained, until the appearance of Trillion, a demon god of destruction that sought to consume the world.

Its power unfathomable, it destroyed the Underworld’s forces. Even Zeabolos, the Great Overlord himself, was defeated.

On the brink of death, Zeabolos heard a mysterious voice from deep within. “It is too early yet for the lineage of the Great Overlord to fall…” Zeabolos was revived, his shambled body patched together again by a young necromancer named Faust.

Zeabolos gained the power of the Soul Grimoire, and decided henceforth to fight.