Nubbeh
Author have you seen the Naruto movie Blood Prison? This film has elements, Ninjutsu and characters that are in touch with the theme of the novel and can benefit the character without leaving the theme. In the film there is a mountruos resembling a biju named Satori that is made of dark energy. The MC could become your host or something. In addition there is another similar Naruto movie where the Biju of zero tails appears made of bad thoughts and negative emotions. Finally the bad guy in the film has a Yang attribute body technique that uses dark energy as fuel. In addition to all this and the movies, there is in a chapter of filler a kind of mansion controlled by a chameleon or vocation, which is 'cursed' and has a spirit inside, the MC could get the invocation that is very powerful in genjutsu and purge the spirit, thus obtaining a mobile mansion completely at his disposal.