| "You have no idea what you've just started."
— Spoilers for Deus Ex Universe: Children's Crusade follow!
The augmented son of Titus King, Sebastian had an illness that required life-saving augmentations some time before the Aug Incident. However, when Hugh Darrow’s signal was broadcasted, he lost control and accidentally killed his mother, Daniela King. This caused a rift in the family, with Titus going on anti-aug rants in his reports and Sebastian eventually going to Glasshütte, Dresden’s aug ghetto, and starting up an aug terrorist faction there.
Sebastian eventually broke out of Glashütte and made his way to Prague where, 2 weeks later, several government officials’ children were kidnapped. When Task Force 29 was sent in to investigate, Adam Jensen came across Sebastian, who had received strength and speed enhancing augmentations since the Aug Incident. Sebastian ended up fleeing the fight and was later seen jumping out of a window to the alley below before escaping.
He later re-appears inside the Glassworks in Glasshütte, where another aug informs him that Interpol is at their front door, unaware that Jensen is listening in nearby. As he orders them to move their stockpile of weapons before Interpol can get in, Jensen throws a concussion grenade into the crowd and in the confusion, Jensen is able to get close to Sebastian and holds an arm blade to his throat. Unperturbed by this, Sebastian calls Jensen a traitor and says that the rest of the augs will bring the war Jensen's "puppet masters" want. He then orders one of his men to kill Emilia, incorrectly believing her mother, Helena, gave them away, before being knocked out by Jensen. However, he somehow managed to escape before TF29 could close in to collect him.
Sometime later, he appears overlooking part of Glasshütte and comments that everything is going perfectly as he watches Titus King broadcast his hatred towards augs right outside of the ghetto’s perimeter, stirring up people on both sides of the wall. He then gets a call informing him that Emilia is going to be moved and he gathers several of his men before making their way to Prague. With help from a mole inside Interpol, they kill Helena inside the car Jensen is driving before causing it to crash, knocking out the agent and Emilia. As two of his men retrieve the girl, two others ask if they should kill the unconscious Jensen, but Sebastian replies no, as that was not part of the deal, so they leave with the girl.
Back in Glasshütte, he is once more standing on his perch, overlooking the ghetto, as he talks to a group of augmented children, telling them the world will never accept them for who they are and they must fight, going as far as telling one boy that he is wrong when the boy wanted out. He later leads the group through the streets, dodging drones, until they come to a building near the perimeter. He has one of the children set a bomb while he arms the rest with an assortment of weapons. Shortly after, the bomb goes off, blowing a hole in the perimeter, and his soldiers pour out, killing everyone before they grab Titus King and drag him back into the ghetto.
Back in the Glassworks, Sebastian has set up his own stage to broadcast from, having had his father tied up to a chair and put in the middle. As he reveals to the world through a hacked feed that he is the anti-aug reporter’s son and why he was augmented in the first place, Titus retorts back that he hates Sebastian because he killed his mother. His broadcast is later taken down by the Juggernaut Collective, who have warned Jensen that Sebastian plans to blow himself and his father up. Trying to end things peacefully by quietly disarming the detonator inside Sebastian, Jensen gets his attention inside the Glassworks but, realizing his backup feed is down as well, Sebastian decides he said what he needed to and orders Emilia to shoot Jensen if he moves. Sebastian then sits on his father’s lap and hugs him as he sardonically talks about how they can be a family again, insinuating he is about to kill them both. Before Sebastian can act, however, he is shot in the head by a sniper, instantly killing him.