The Demon Master is an antagonist seen at the end of Quest for Glory 3 by all characters except (for an unknown reason) a Paladin.
The hero (unless he's a paladin) attacks the Demon Wizard from behind, but the Demon Master warns him about it (even though a Wizard might have already hit the former by then). But in the case of a Paladin, the Demon Wizard already faces him in advance (for an unknown reason).
Background[]
This Demon is the Demon Wizard's Lord and master. He appears in the World Gate to Hell when the Hero enters the final chamber of the Lost City in order to confront the Demon Wizard.
If the hero waits long enough it's explained the Demon Master entered the world and destroyed everything with Thermonuclear Blast.
Behind the scenes[]
The name Demon Master originates from a death scene, the Demon Wizard just calls the being his "Lord".