Mard Geer Tartaros (マルド・ギール・タルタロス Marudo Gīru Tarutarosu) was an Etherious and founder of the Dark Guild Tartaros. He was known as the Underworld King (冥王 Meiō) and the Definitive Demon (絶対の悪魔, Zettai no Akuma), having placed himself in charge of the guild in E.N.D.'s absence.