Sethius was an ancient vampire who, according to Count Dracula, was around 3000 years old by the time of his death ("Hit Chicks"). According to legend he was one of the very first vampires. He was also the self-proclaimed Chosen One of vampire-kind. His plan was to turn every breather in the world into vampires and begin a vampire civil war, but the ancient vampires believed he was insane and imprisoned him in the Praedictum Impaver. The Praedictum Impaver eventually fell into Bertrand's hands. Bertrand believed that once opened, the book would reveal the destiny of the vampires.
Bertrand travelled to Garside Grange to train Vlad, the true Chosen One, in the use of his powers so he could open the book. He eventually does so, but finds the pages blank. Erin later fills them with her blood and Sethius is freed.
Sethius recruits Ingrid and Bertrand as lieutenants and sets about planning to conquer the world. Once again he tries to convince them all into following his insane plan to turn every breather in the world into vampires and begin a vampire civil war. Vlad tries to defeat him but Sethius appears to be even more powerful than him (or at least more skilful in the use of his abilities). Eventually Vlad convinces Ingrid and Bertrand to betray Sethius, and they (together with the Count and slayers Jonno and Mina Van Helsing) attack Sethius, distracting him until Renfield and Wolfie are able to destroy him with a Suncatcher machine.