因果律の花 (Ingaritsu no Hana)

Background
This song was released on If the World and is based on the fairy tale The Black Princess.

A young princess (Rin) has a beast inside her castle that awakens every night and kills anyone it comes across. One day she meets a young man (Len) and falls in love with him. She tells him of teh beast and he vows to slay it. He takes a sword and goes into the place where the beast sleeps but after he stabs it, he realizes that the beast is actually the princess' herself, she is briefly returns to normal before she dies.