Alluring Secret

I Do Not Own: This is a novel adaptation of an vocaloid songs "Himitsu ~Kuro no Chikai~".
This is about Rin, a young angel who fell in love a young (human) woman named Miku. However, their love is considered a taboo due to both their gender and race, and so, Rin makes a deal with the devil. To be reborn as a human boy, Len, in order to be with Miku. But their happiness is short-lived, and soon ends in tragedy...