Baelfire" The Devil" is the main antagonist. He is the destroyer of both worlds.
Bael use to be the one of the shadowhunters. He turns against them for not agreeing with him about making more hunters. Bael now rules the underworld along with his friend Hades. Bael betrays Hades. He takes Hades powers and locks him in the dungeon. Now Bael rules the underworld and the demons