Deimos no Hanayome

A deity who was once handsome fell in love with his sister, a goddess who shared his feelings. Their relationship was considered unnatural, leading to their punishment by being cast from Olympus. The brother became a demon, while the sister transformed into a decaying body resting at the ocean's floor. Now, Deimos must decide between his sister and a living human girl who embodies her. His sister feels resentment, while the girl is frightened and uncertain about her circumstances.