Sebastian was created on 3 March 1981 by Elena Hilda Al-Qatari. He had been created to be a soldier, but he had no killing instinct. He learned quickly, being able to read and write several languages in a few days. Jack Harkness was sent to monitor his progress and train him in weaponry. In 1991, the Juniper Project sent in soldiers to train Sebastian properly. If he didn't fight, they beat him. When Jack Harkness learned what they did to him, he broke him out of the faculty and brought him to Rawbone. There, they set up a facility to create Scions as children for the town. Sebastian worked there for the next 2 decades. In the meantime, a second Sebastian had been created with a killing instinct. Eloise tried to avoid activating the second Sebastian, believing it would be too dangerous.
In 2011, Gwen Cooper, Rhys Williams and Anwen Williams came to Rawbone while on the run. The presence of a real human child disrupted the project, turning the people against the Scions. After Billy was nearly killed by his mother and Jenny stole Anwen to try to become a real mother, the leaders of the Juniper Project decided to accelerate their plans. Eloise was forced to kill the original Sebastian and awaken the second.
This new Sebastian had no respect for humanity and decided to take control of the Scions. He used them to kill Billy's mother and then planned to kill all the other people in town. When Eloise and Gwen tried to stop him, he locked them up. They were able to escape and used the Juniper Tree to contact the aliens who created it. He believed that they would be pleased with his attacks, but they were actually appalled. He tried to create more Scions, but the aliens killed them and the Tree, stopping him from creating more. Sebastian blamed Gwen for this and tried to kill her, but she was saved by the other Scions, who had been released from his control. They turned against him and killed him. (PROSE: First Born)