Archduke Sebasis was first seen in Season 5 of Angel. He was seen when Lorne is creating the Wolfram and Hart and Lorne forces Angel to invite him to the party because he is one of their biggest clients. Sebassis is sure it is just a trap set by Angel to kill Sebassis since he is the leader of the Circle of Black Thorn. Thus he brings multiple demon guards heavly armored. turns out Lorne had the sorcerrers at Wolfram and Hart remove his sleep thus causing him to make a double of himself with enhanced stregth. Angel saves Sebassis by restioring the double into Lorne. 
 
   Months later Sebassis was part of the Circle of Black Thorn that tortured Drogyn the Battlebrand and helped bring angel into the Circle. Later, when Angel and team divided into groups to defeat the Circle, Angel got Sebassis to take care of. Turns out Angel posioned Sebassis demon whose blood he drank, thus posioning Sebassis.