Could you do my Fallen Stars au in this? Human souls are rocks. Chara's soul is a black hole. Monster souls are stars. Monsters have a star birthmark on their fur/skin/bones/scales.
Hmmm... in my universe Chara, or as we call it-yes, it- The Blackhole, has a mass of dark matter for a soul, which, uppon their first kill, absorbs the dust and Extermination Points and becomes a black hole.