This is the game's "secret" boss. It is the Demon King that you prevented from being completely revived in Path B of the Forgotten Sanctuary. You fight him at full power on floor 9-1 of the Labyrinth of Chaos. Within the game's story, he is responsible for giving the Dragon's Crown the power to control dragons in exchange for Wallace's soul until the player recovers the stone tablet contract and gives it back to Wallace, who destroys it. The Demon King's real name is Majino-Gusna-Idra-Shin.
Stack as much +/- % Damage and +/- % Damage to/from Demonic enemies as you can. Immunity to Petrification is HIGHLY recommended due to his triple gaze attack that comes from random locations on the screen. While it is possible to evade these attacks, having immunity will let you focus on dealing damage. Note that you just need to turn your back against the eyes of Medusa and then you wont be petrificated.
He has attacks of all elements at his disposal. His ice attack hits hard, his lightning attack stuns, and his fire attack covers a large area. Occasionally he will spawn a purple circle on the ground...