Mallen used to be an agent of S.H.I.E.L.D. But he was fired for his unstable agressive behavior (he assaulted Agent Dugan for an unknown reason). Out of spite, he stole their experimental super-soldier serum named Extremis. He injected himself with it and was encased in a black chrysalis. When he emerged from it, Mallen was transformed into a mutated human. He started assaulting S.H.I.E.L.D. agents and terrorized the city. Iron Man suited up to stop him, but Mallen was too strong and mercilessly beat him to near-death. Mallen was driven off by S.H.I.E.L.D. air vehicles and took Iron Man back to the Helicarrier for medical treatment.
Mallen started having hallucinations about Nick Fury firing him. He went on another rampage and was confronted by Black Widow, Nick Fury, and about four Mandroids, all of whom were easily beaten by Mallen. He almost killed Fury, but Iron Man (who has taken a drop of the Extremis serum) saved him and is back for a rematch. Iron Man's new enhanced reaction time and cyberpathic connection to all machines allowed him to handle himself better against Mallen. Iron Man used a ground wire to electrocute him into a coma. He was then hospitalized by S.H.I.E.L.D.
After taking the Extremis drug, his biology was enhanced. Mallen was transformed into a superhuman being. He gained the following abilities:
- Superhuman Strength: Extremis modified his body and enhanced his physical strength, making him able to toss cars and people with relative ease.
- Enhanced Speed: He is also incredibly fast and agile, able to run at really fast speeds. Mallen could enhance his speed through his electricity.
- High Durability: He also has great endurance. He can shrug off bullets and attacks from Iron Man. It may have also provided him with a nearly-unlimited stamina.
- Nigh-Immortality: Mallen can instantaneously regenerate on a cellular level, which is the main reason behind his apparent immortality. An example if his seemingly immortality is him being able to realign his broken neck the second after Black Widow twisted & broke it off.
- Claw Pincers
- Electrical Discharges: Mallen can emit and project destructive lightning arcs strong enough to damage surrounding city blocks, knock back cars and S.H.I.E.L.D. air vehicles. He could even focus them into shock blast rays for a more concentrated attack.
- Fire Generation: He can generate intense heat and flame either by exhaling flares of it or emiting focused beams from his hands and mouth.
- The Extremis serum affected his mind, giving him frequent hallucinations and worsening his unstable mentality.
- Nick Fury once threatened to "knock his (Mallen's) skull off of his shoulders". It's ironic because that is exactly how Iron Man stopped him in the comics. He was forced to blow his head off with his repulsors, killing him.