The Mind-Monster was an extradimensional monster and an opponent of Captain Britain - the mystical defender of the British Isles in Marvel comics.
Captain Britain's body was lying on a hospital bed at S.T.R.I.K.E. HQ, near death following an encounter with Lord Hawk. His spirit however was unexpectedly drawn from his inert form into an unknown dimension, where he was confronted by a giant, fire-breathing monster. The hero was unsure as to whether the creature he saw before him was real or a dream, but either way the thing was charging towards him. For a few seconds, Cap was paralyzed by fear, but then he gathered his nerves, and decided to fight. He narrowly dodged the behemoth's fiery blast, but lost his footing as the ground shook under its thunderous tread. Dropping his staff, he barely avoided the beast's huge fist, which instead smashed into the ground behind him, sending rocks flying. Captain Britain's leg was trapped amongst the falling stones, pinning him as the monster lunged forward, while Cap's body back in the real world mirrored his injuries, with a cut appearing on his leg. In another dimension "far beyond our reach or comprehension," Merlin and the lady Roma watched the duel with trepidation.
The two mages observed as the creature loomed over the trapped hero. Roma plead that Merlin should spare the hero, but Merlin disagreed: Captain Britain "must yet survive this final test -- which seems most unlikely!".
Back at the battle ground, the flames from the monster's mouth engulfed the stricken man, scorching his very soul. In spite of the pain, he resolved not to give in, and then realized that although he felt like he was on fire, his body remained unharmed. Once more the hero wondered if he was in the grip of a nightmare. His eldritch observers continued to argue as to whether he had yet proved himself, until Merlin relented a little, stating that Captain Britain had now displayed enough courage to grant him some concessions, and that he would return to him his weapon. The staff materialized in front of the trapped man, who grasped it and activated its force field. Shielded now from the flames, he was spared the agony he had been feeling, but with the monster still towering over him, realized his nightmare was not over yet. He was wrong about this however, for as the creature started to pound away at the protective shield surrounding Cap, Merlin decided that he had passed the test, and summoned him to Otherworld, leaving the Mind-Monster without a target for its rage.