Sett is a reconstituted demon, summoned by Incognito using Integra's blood. It's never explicitly stated, but as Tubalcain, it is implied that he is Seth, the son of Adam and Eve. Though referred to as a demon in the dub, in the Japanese version Integra refers to it as "Jashin Seto", or "The Wicked God Sett", implying this heavily; Incognito later says he wields the power of an Ancient god as well, referring to Sett. However, considering Ingonito's connection to the 'Dark Continent' (better known as Africa), this might be refering to the Egyptian deity known as Set.
Sett appears as a streak of white light, moving through London destroying everything in its path, and was absorbed into Incognito's body as part of his ritual. Sett also breaks the boundaries of Dark Magic, meaning he can increase the user's power to maximum allowing them to break their Dark Magic limits. Incognito used its power to fight Alucard in his Level 1 form, but it proved not enough and Incognito was killed. Sett's fate after that is unknown.
Although a God, he is still too weak to kill Alucard