Himitsu: Kuro no Chikai

Rin, a young angel (presumed female, but angels don’t have clear genders) falls in love with a young human girl, Miku. However, their love is considered taboo, and so, Rin makes a deal with the devil to be reborn as the human boy, Len, in order to be with Miku. However, their happiness together is short-lived, and even that soon ends in tragedy.

(Synopsis: NovelUpdates)

Himitsu: Kuro no Chikai is an adaptation of the Himitsu series, a trilogy of songs by Hitoshizuku-P and Yama△.