After 9/11, Sebastian was sent to serve in Afghanistan by his father, Nigel. While there, he became an experiment of Muirfield. When the experiment was eradicated, Sebastian survived and went into hiding, faking his death. He held Nigel responsible for what happened to him, and was intent on getting revenge against him.
When Agent Reynolds starts sending Vincent on missions to hunt down any remaining beasts by, he gets the help of Agent Tucker to gather information and keep tabs on Vincent. Tucker finds out about Sebastian still being alive, but Sebastian figures out he's being followed. He follows Tucker to his car just as Tucker is sending information about Sebastian to Vincent. Sebastian tries to get him to give up who sent him, but Tucker doesn't talk, and Sebastian brutally kills him, which Vincent overhears.
When Vincent goes to investigate the crime scene, Sebastian is waiting for him, and uses his magic tricks to evade Vincent. He also lets Vincent know that he's also on the hit list of beasts who he was ordered to exterminate. Sebastian disappears, but Vincent is determined to find him.
Nigel hosts an auction of his private art collection soon after, with the main piece being "Friday at Five", a painting he commissioned as a tribute to Sebastian. Vincent shows up to the auction, as does Catherine Chandler. Noticing the two together, Sebastian targets both her & Vincent. Before Sebastian can pull off a grand entrance at the auction in order to shock his father, Catherine's colleagues evacuate the room, and leave Vincent to deal with Sebastian.
Catherine gets Nigel into the elevator to keep him safe. Sebastian manages to knock Vincent out, then goes after his father. Catherine is waiting, though, and tranquillizes Nigel. She tries to tranquilize Sebastian, but he grabs her. Catherine tries to fight him, but he tries to drop her down the elevator shaft. Vincent shows up, though, saves Catherine and pushes Sebastian down the shaft, killing him.