Fiend

A Fiend (魔人) (literally: Jinn or Human Devils) is a devil which has taken over a person's corpse.

Overview
Fiends are human corpses which are taken over by devils as it is their last resort to survive. Devils lose significant amount of their supernatural powers and ability to perform contracts after becoming a fiend. Fiends can be identified as fiends due to their heads having distinct shapes, such as Power's horns, Beam's shark head or Pingtsi's hair.

Fiends have the personality of the devils which took over their corpses. However, some fiends retain more of the original corpse's brain than others, leading to their personalities becoming mingled. The only known case of this is the Violence Fiend who should be inclined towards violence but instead prefers peace.

Abilities
Fiends possess the supernatural abilities of their respective devils but in a lesser degree. Some fiends are able to transform into their original devil forms temporarily. They also can restore their health by consuming blood. However, they are unable to make contracts with devils or humans either.

Trivia

 * The Jinn (جن) is Arabic for hidden being and they are either spirits or devils, often translate in English as Fiend, Demon, Daemon, Genie, Spirit, Ghost and in Japanese as Majin (literally "Magical Person"). The word "Majin" can also be translated as "human devil".
 * Unnamed fiend is described as Bird-Eating Fiend (鳥を食べていた魔人 Tori o tabete ita majin) in the second Character Popularity Poll.