Mephis SaDiablo was the eldest son of Saetan SaDiablo and Hekatah SaDiablo and older brother to Peyton SaDiablo and a half-brother to Daemon Sadi and Lucivar Yaslana. During the first war between Kaeleer and Terreille Mephis was killed and became Demon Dead, about 50 000 years prior to the events of Daughter of the Blood.
Mephis worked in the Dark Realm, also known as Hell, alongside his father, Saetan. His work consisted of taking care of the realm, and running it when his father's not there. He also ruled Dhemlan Kaeleer during his father's absence.
During the second war between Kaeleer and Terrielle, Mephis volunteered to return to the darkness with other demons, in an effort to keep the "dream" alive and cleanse the territory of the corrupt Blood.