Kishin

Kishin (鬼神, kishin; FUNimation "Afreet"; Literally meaning "Demon God") are beings created as the final result of a corrupted soul. A soul becomes corrupted when it devours pure human souls to gain power, usually as a means to escape fear. As a result, the person's soul deviates from the path of a human and begins to walk the path of a demon, becoming a "Kishin's egg" (鬼神の卵, Kishin no tamago).

HISTORY (In anime)
The first kishin created, Asura, was ironically part of the group which was lead by Shinigami himself to rid the world of corrupted souls, the Eight Powerful Warriors. Out of fear, Asura ate pure human souls that were not part of Shinigami's list. Asura became so fearful, that he would soon eat his own partner weapon, Vajra. Asura was eventually stopped by Shinigami and had his entire skin removed, his blood drained and used the same skin as a sack for his body. As a result of this, Shinigami rooted his soul to Death City to keep Asura sealed and founded Shibusen to prevent the creation of any new Kishin.