Sebastian is an Undead Vampire Spawn who is imprisoned in Cazador's Dungeon.
Interactions
As the party progresses through Cazador's Dungeon, they inevitably pass by a prison containing Sebastian. Astarion starts to comment on the prisoners when Sebastian recognizes his voice-- the voice that lured him into Cazador's clutches. He recounts how they met in a tavern, got drunk, and romanced each other, before lunging at the bars in a desperate attempt to take his anger out on Astarion.
As Astarion and the party try to reckon with this information, Sebastian quietly asks how long he's been imprisoned. Astarion informs him that it's been 170 years and he sobs as he realizes that all his family and friends are long dead. The player character can then promise to free him or tell him to stop whining. If they ask him how to free the prisoners, he informs them that Cazador's staff Woe is the key.
If the party chooses to free the imprisoned vampire spawn after killing Cazador, then Sebastian will escape and send them a letter during the epilogue party.