"Mars will never be free until the sands run red with Earther blood!"
- – Abel Horn
In his youth, Abel Horn was said to be a "bright young man utterly devoted to the independence of Mars." By 2244, he was a member of the fledgling Free Mars, a group dedicated to a free Martian state. During that time, he met Amanda Carter and convinced her to join the group. Within a few years, however, the group turned violent, becoming a full out terrorist organization. At that time, Carter and Horn parted ways.
Over the next decade, Horn was rise to become one of the leaders of Free Mars and one of its most violent members. He eventually became the suspect in no less than 75 criminal incidents.
In 2252, he was sentenced to Phobos Maximum Security Outpost for his involvement in the computer theft of over 50 million Conglomerate credits. After serving one year, he was transferred to the Lunis Planum Prison Base, though he escaped after just three months on April 16, 2253. Not long after that, Horn orchestrated the bombing of Ritchey Station, a major incident with many civilian casualties. Horn publicly claimed credit for the bombing.
In 2258, during the Mars Rebellion, his ship was attacked by the EAS Pournelle. The ship's hull was ruptured and Horn died. His body was recovered by Bureau 13 and used for the Lazarus Project, wherein Horn was "resurrected" as a kind of cybernetic assassin with no real mind of his own. He was then sent to Babylon 5 to murder Taro Isogi (though the exact reasons why are unclear). He completed this task, but contact with a telepath, Talia Winters, triggered some of his memories—enough for his original personality to briefly assert control. Desperate to find out what had happened to him, he discovered Amanda Carter was on the station. He approached her and asked her to arrange for him to meet Talia privately. After being lured by Carter to the meeting, Talia managed to probe Horn's mind and uncover the truth. Horn was subsequently shot and "killed" (or disabled) by station security. His body exploded immediately afterward to conceal any evidence of the Project.