Demonic Body

Demonic Body (悪魔のような体,? Akuma no yōna karada) is Form of Forbidden Magic where user fuses his body with demon or Demons to get their powers. In exhange, they lost part of their humanity and mortality. However, if Demon manages take over caster, user turns to demon and loses himself.

Description
More Curse than Magic, Demonic Body was created to "Seal" Demons away to living hosts. It however was too dangerous to use, so it was classified as "Forbidden Magic" soon after creation. Even after this some mages kept using it, and ended up being devoured by demons from inside. Two most notable succesufull users of this Magic are Raven Theus of Ebon Wing and former Mage of Dark Guild Daemon Blade Luce Nox