The Extremis serum is the chemical basis of the entire Centipede project, the concoction which provides superhuman abilities to human subjects. It was developed by Aldrich Killian, which gave its host the ability to heal & regenerate physical & mental damages instantly, including limb & organ regeneration in a few minutes. It even provides its host with various superhuman physical enhancements and an exothermic ability. It has since been modified by the Centipede Group to its current Stage Two level. Agent Phillip Coulson would appear to be the key to Stage Three, specifically his death and revival.