Real name Douglas Birely, Doctor Demonicus is a geneticist who dedicated his life to the effects of radioactivity and the resulting mutations. From his work, Birely was exposed to a large amount of radioactivity himself, which caused him to contract a rare form of cancer. When this became known to his colleagues he was fired from his position. Filled with malice, the Doctor set up a laboratory on the Aleutian islands. On the isles he discovered a fallen meteor called the lifestone, which allowed him to create rapid mutations in other organisms that Demonicus was then able to bend to his will. The Doctor also managed to create a special suit that helped to subdue the effects of the cancer he was carrying. With his new found power, Demonicus also hired a sizeable militant force, dubbed Demon-Soldiers. Stock piling mutated creatures, the doctor stood unopposed on the island chain. Unfortunately for Demonicus, one of his pets, Batragon, brought back an unexpected guest to his hideout: Godzilla. As it turned out, S.H.I.E.L.D. (Supreme Headquarters International Espionage Law-enforcement Division) was also in pursuit of the nuclear leviathan. This led them to the Doctor's stronghold, where they witnessed the defeat of various creatures such as Ghilaron and Centipor. Thanks to the efforts of Godzilla, S.H.I.E.L.D. was able to contain Demonicus and destroy his lifestone.

Regrettably this did little to curb the Doctor's criminal activities, as once free he used an electromagnetic beam to try and drive an asteroid into the Earth. His efforts were again thwarted, though, by the combined might of S.H.I.E.L.D. and the Shogun Warriors. Still, despite setbacks, Demonicus continues his schemes. His next plan saw the Doctor create an undersea hideout in South Pacific. To compliment his base, the fiend also managed to capture the once King of the Monsters. With Godzilla in his grasp, the Doctor began to mutant the creature further as he was slowly bent to the madman's will. Once tamed, Godzilla was kept as a watchdog to the undersea base. As fate would have it, the Avengers Quinjet happened by the area and was attacked by the nuclear leviathan. This brought other Avengers to the island, particularly Iron Man who would later fight Godzilla to a stand still. Even in his more primitive state, the King of the Monsters still had enough intelligence to carry off the old Iron Man suit when Tony Stark discarded it to save himself from drowning. Godzilla then presents this to Demonicus, who quickly equips the suit and flies off only to be soundly defeated. With the Doctor taken into captivity, Godzilla escapes never to confront Demonicus again... but the loss of his pet and base again does not deter the man's criminal intent.

Eventually, Demonicus becomes free again and this time begins to mutant humans to do his bidding. He is again defeated, though, by the Avengers. After this, the true effects of his cancer are also revealed, as once his mask is removed it's shown that his mouth has mutated horribly as tiny skin flaps connect his two lips. Two horn-like skin structures have also formed on the Doctor's forehead. This last setback ends up being more permanent, as the Doctor is imprisoned for a considerable amount of time, eventually being moved to the prison confinement called The Raft. The madman is freed, though, during a large scale breakout created by Electro, which allows Demonicus and many other super villains, such as Carnage and Sauron, to escape. Currently, Demonicus is still at large.