Doctor Demonicus, real name Douglas Birley, is a recurring character in the Marvel universe, who first appeared as a foe of Godzilla.
Douglas Birley was a geneticist fascinated by mutation, which he did studies on. After being contaminated by a radiation spill, he went mad when he realized he did not gain superpowers, but instead got skin cancer. Without a job, Birley saw a report of a meteorite crashing in the Aleutian Islands, which he took over with his army of Demon-Soldiers (former mercenaries and criminals) and enslaved the native Inuits who lived there, and planned to use the meteorite- now called the Life Stone- against the world he feels had betrayed him.
Inspired by the giant monster Godzilla, Birley, now Doctor Demonicus, began to subject the tissues of various creatures to the radiation of the Life Stone. He then extracted the hormones from the mutated tissues, then injected them back into the creatures, and exposed them to the meteorite. The creatures mutated, the first of which became Batragon, and then the trio of Centipor, Ghilaron and Lepirax.
He sent Batragon to attack a Liberian oil tanker so his Demon-Soldiers can get the oil for him. After Godzilla fought and wounded Batragon, the creature retreated back to Demonicus' base for the Life Stone. Demonicus ordered the Inuits to continue their work or he'll use his monsters against them. Batragon was eventually killed and Demonicus prepared to use the rest of his creations against Godzilla.
S.H.I.E.L.D. found the other monsters, and Demonicus sent Centipor, Ghilaron, and Lepirax against them. Gabe Jones made it in Demonicus' presence, but Demonicus held him at gunpoint. Demonicus get's distracted by Centipor's death and is defeated by Gabe Jones, and is put into S.H.I.E.L.D. custody, while his remaining two monsters were slain.
Some time later, Doctor Demonicus confronted the Shogun Warriors and was arrested by S.H.I.E.L.D. again. After escaping custody, Demonicus managed to capture Godzilla and mutate him. Demonicus was in the process of creating more monsters, when Godzilla downed an Avengers Quinjet that was piloted by Tigra. Not wanting to attract any attention, he ordered Godzilla to destroy the aircraft and the superhero. Tigra called for help, and escaped the craft. Demonicus then made Godzilla and some of his Demon-Soldiers to attack a nearby village. With Hawkeye and Mockingbird stopping his soldiers and Iron Man, in his Mark I armor, taking Godzilla out to sea, Demonicus escaped.
Godzilla brought Demonicus the empty Mark I Iron Man armor that Tony Stark abandoned, and Demonicus wanted revenge on the superhero, putting his other plans on hold. Using the newly-acquired armor, Demonicus traveled to California to investigate reports of Iron Man Mark VII armor there, and found the armor at Tony Stark's base. Believing this armor is piloted by Iron Man, (when in reality, it is being controlled by the extradimensional entity Omnos) Demonicus attacks it. Stark activates the magnetic circuitry in both suits of armor, locking them together. Demonicus was defeated once more.