Himitsu: Kuro no Chikai

Rin is a young angel whose celestial nature transcends conventional gender, though she is generally perceived as female. Despite the rigid boundaries separating the divine and mortal realms, Rin finds herself deeply captivated by Miku, a human girl. Their profound affection defies the natural order, rendering their bond an absolute taboo in the eyes of both heaven and earth. Driven by an unwavering devotion to Miku, Rin makes a desperate and forbidden pact with the devil. The terms of this dark agreement demand a complete surrender of her angelic existence. In exchange, Rin is reborn into the mortal world as a human boy named Len, finally gaining the ability to stand beside the girl she loves without the constraints of her former celestial form. For a fleeting moment, Len and Miku experience the joy of being together, living out the love that once seemed impossible. Yet, the price of defying fate proves far steeper than anticipated. Their hard-won happiness is tragically short-lived, as the consequences of Rin’s infernal bargain soon unravel their lives, plunging their story into an inevitable and devastating tragedy.