Kegare no uta

In order to deal with karma demons, monsters which devour people and become corrupted by "taint," villages call Taintspawn, humans who choose to kill karma demons, although it means absorbing the "taint" of their prey. Torie, a young bard struggling to compose songs comes across an awkard Taintspawn named Ran during her travels and decides to travel with him in order to write a song.

(Source: MU)