Childhood friend (♀) gets attacked by something (♀), Main character (♂) dies in crossfire. Main character (♂) is reincarnated as the living weapon (♀) of the demon lord (♂->♀) that just killed him, while his childhood friend (♀) is apparently now the hero (♀->♂?). Reincarnation + Isekai + Double(?) Genderswap.