Demonicus, known as the Demon early on, spent his time practicing the black arts in a locked sub-cellar located in . He first appeared on the villain radar when his disciple abandoned him and went to Dr. Strange for help, believing only he could stop his evil master. When Demonicus became aware of his disciple's betrayal he transported his body sans clothing back to his lair. Not one to ignore a cry for help Strange set about locating Demonicus. He used his powerful amulet to animate the disciple's abandoned clothes to show the way from which they came.
Sensing the arrival of Dr. Strange Demonicus readied a powerful trap using the crimson bands of Cyttorak. After capturing the Ancient One’s pupil Demonicus tried to obliterate him, but nothing he tried would work. Strange, meanwhile, used his astral self to search for the spells that Demonicus had mastered. Once he had this knowledge Strange was able to bust free of his crimson prison. Demonicus then engaged him in a mystic battle, but despite the onslaught of sinister spells Dr. Strange defeated him. Afterward, Demonicus was placed into a mystic trance where Strange tried convincing him to renounce his evil ways.
The conditioning on Demonicus didn't last long, probably due to his allegiance to Mordo who had ordered his minions across the globe to pursue Dr. Strange. It was in the Sanctum Sanctorum that Demonicus was stationed awaiting a possible return from the master of the mystic arts.
One day Demonicus had a visitor come to the door wanting to challenge Dr. Strange in a mystic brawl. Demonicus dismissed him as some crackpot and when he let down his defenses the weirdo clocked him across the jaw and knocked him out. The mysterious person turned out to be Dr. Strange who had purchased a costume so as to not be recognized and have Mordo alerted to his return to .
Demonicus wasn't seen again until he teamed up with two of Mordo's more powerful allies, Adria and Kaecillius. Together the trio took out Dr. Strange, shackling his hands and placing an iron mask over his face. Demonicus was tasked with probing Strange's mind, but the adept Strange easily turned the scales making Demonicus his mind slave. Strange forced him to break the spell holding him in place, but before he could be unshackled the powerful mystic Adria sensed what was happening and placed Demonicus into a stony trance.
Eventually Demonicus recovered and led the search party for the missing Dr. Strange. After hours of fruitless hunting Demonicus went to deliver the news to Adria in person and found her engaged in battle with Strange's astral self. He joined the fray and it seemed as though he and Adria had the good doctor on the ropes, but a timely maneuver from Strange resulted in Demonicus getting nailed in Adria's crossfire. Strange was then able to place Adria under his control and had her remove the restraints from his body. From there he erased the trio's minds of all black magic in the hopes they would someday redeem themselves.
Instead, Demonicus and his two black magic partners tried one more time for revenge on Doctor Strange. Posing as an interview team they were invited into the Sanctum Sanctorum to learn of Dr. Strange's origins. A battle ensued in which Adria tried using the powerful Purple Gem against Strange. However, her plan backfired as Demonicus, Adria and Kaecillius were sucked into the Purple Dimension never to be seen again.