The Grim Adventures Of Billy and Mandy (2001)

After Grim's robe comes out of the washing machine, Billy and Mandy find Thromnambular, a talking skull with ruby eyes and a number nine imprinted on his forehead. Grim tells them that the skull is a magical artifact imprisoned on Earth that must grant his nine wishes to be free. When Mandy asks Grim why he has not used any of the wishes, Grim tells her that the skull will corrupt the wishes and end up affecting the wisher.