 If you liked
Kiseijuu
|
 ...then you might like
Black Torch
|
Both manga have main characters who were saved by some supernatural being on the brink of their death, saving him and making him way stronger than before. This being still has an own will after merging with him, and can (sometimes) leave his body as he pleases. They are caught up in all sorts of shady experiences.
*Both of them often have gore scenes.