Seth is Malvel's most trusted minion. He appeared as the major villain of the Dark Realm series(series three). Sethrina is his older sister. She also works for Malvel.
Seth appears in Skor to fight Tom in order to help Skor and kill Epos. In Tusk the Mighty Mammoth he also appears. In Sting the Scorpion Man, he was turned into Sting, the Scorpion Man, as punishment for failing to defeat Tom. He was (apparently) killed when Malvel's castle fell on top of him.