|“||Other worlds... Why should I care about some other world when I rule this one? What should some other world mean to me?||”|
- This character's history and characteristics are identical to that of his alternate dimension counterpart up until just before the Dark War, where the timeline split and the following changes took place.
At the Seventh Sacred Site, Sebastian, with Jace, and Clary by his side, made preparations for the creation of his Endarkened army. He summoned Lilith who supplied them with the demon blood for the Infernal Cup. The Shadowhunters arrived shortly after. Clary, armed with Glorious with the intent to free Jace from Sebastian's hold, was killed by Lilith. To maintain control over Jace, Sebastian told him that he did not want Clary's death and that he apparently murdered Lilith in retaliation.
After a few months of war and hostile takeovers, Sebastian won, having turned many Shadowhunters into Endarkened and many Downworlders and mundanes into submission, forcing others into hiding. Sebastian opened doorways into their world for demons, and soon the world was overrun with them; the doorways also affected warlocks and angelic magic. With the threat of the Clave no longer around, and with his life tied to their world—meaning no warrior of Thule would ever be able to slay him—Sebastian freed himself and Jace from the aspect of their bond that bound their lives together, allowing one to die without killing the other, in part to protect himself in case anyone ever attempted to kill him through Jace.
Around two years after his victory, two people from another dimension arrived: Annabel Blackthorn and Ash Morgenstern, the latter of whom is the child of his counterpart in their world. Sebastian took him in and treated him like his son—at least, he did so to the best of his ability, not exactly knowing how it is to love anyone.
Sebastian's efforts to take over the world continued for half a decade, but despite all odds, he was still met with some resistance. At one point, two more people from Ash's dimension arrived: Julian Blackthorn and Emma Carstairs. Though the pair pretended to be their Endarkened counterparts in Thule, he recognized that they were imposters but did not let on and instead allowed the two to return to the rebels, hoping that they would be able to open the heavily sealed Silent City. Sebastian hoped to get ahold of the remaining Mortal Instruments, believing it might allow him to finally gain control over the resisting Nephilim. Sebastian and his followers lay in wait outside the Silent City, knowing that they were close to finding a way.
Meanwhile, years of controlling the Endarkened took its toll on him, gradually making him weaker. Both Annabel and Ash realized this and felt that he would someday turn on Ash and kill him to regain his strength. Because of this, Ash betrayed Sebastian during the siege of the Silent City. Ash used his power on him, effortlessly making Sebastian bend to his command; Sebastian let Ash take his weapon from him during his fight with Emma, allowing her to kill him with the Mortal Sword.
Sebastian was just as ruthless and evil as his counterpart, if not more. He ruled over a world filled with demons and death, not hesitating to kill whoever stood in his way and whoever refused to submit to him. Owing to his demon blood, upbringing, and experiences, he cared only about himself and did not give a thought even for his subjects and followers. He did not mind that he ruled over an empty world, wanting only complete destruction and obedience.
Sebastian was tall and muscular and had a slightly slender frame. He had light skin, made paler with years with no sun, platinum blond hair, and black eyes due to his demon blood. As he aged, Sebastian's face and features became sharper than before.
- Phaesphoros: A Morgenstern family sword once owned by Valentine Morgenstern. It is part of a pair, with the short-sword Heosphoros held by his yes man Jace.
- Queen of Air and Darkness (first appearance)
- Forever Fallen (appeared in flashbacks; mentioned in main story)