Nightmare Sans is a fan-made character from the Undertale alternate universe community, known for his dark and menacing appearance and tragic backstory. Originally a kind and selfless skeleton named Dream Sans's brother, Nightmare was transformed into a corrupted and evil version of himself after absorbing negative energy from a mysterious, cursed apple. Now twisted and driven by hatred, he leads an army of dark beings with the goal of spreading negativity and despair across the multiverse. Despite his villainous role, many fans find his story compelling due to the emotional depth behind his fall from grace and the lingering traces of the person he used to be