Stannis Baratheon is the Lord of Dragonstone and the eldest of the two younger brothers of the late King Robert Baratheon, the other being Renly Baratheon. Because of rumors that Robert's heir Joffrey is a bastard born out of an incestuous affair, he has claimed the Iron Throne and proclaimed himself the rightful King.  He has powerful dark magic on his side.  Whether he can control that power or become manipulated remains to be seen.