He IS a Sith. Well, at least that´s what I think. According to the Official Site, "It`s a non-humanoid alien who likes to hunt jedi for sport", but I don´t believe this crap.

Besides, Canon-fanatics always love to talk about the Two-Sith Rule. Two-Sith Rule my ass. Darth Tyrannus is a Sith, and he was surely very much alive when Darth Maul was Darth Sidious´ sidekick.