Magnus, also known as The Magister, is a vampire judge who presides over all the vampires of North America. He is from the Iberian peninsula and was turned during the late ninth century. He was feared because of his political power and advanced age.
During the first season, he tries and sentences Bill for murdering Longshadow, a fellow vampire. As punishment for the crime, he forces Bill to sire Jessica in order to replace the vampire he's killed during a time when their numbers are so important. In season three, he tasks Sophie-Anne and Eric with finding out who is selling vampire blood and their source.
Although he is suspicious that they are the actual culprits he nonetheless takes their innocence at face value. In episode 7, he is forced by Russell to officiate a marriage between Russell and Queen Sophie-Anne. After the deed is done, Russell uses Magnus' cane to decapitate him.