The Demon of Darkness is a Lesser God. He is summoned by walking up to his Summoning Altar in Hell.
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To get to him the first time, the player must travel North, past the Cerberus, cross the bridge to the north, and open the chest containing a Hell Dungeon Key, then travel through the door to the South. After defeating him, the player can teleport out of Hell, and use the key to open the gate to the West of the entrance on subsequent trips to easily access him.