Zatch Bell!!

Every 1,000 years, 100 demon children are sent to Earth to compete in an epic battle to become the king of the Demon World. Each demon pairs with a human partner who wields a spell book of a unique color, enabling them to cast powerful spells. If a demon’s book is destroyed, they vanish and return to the Demon World. The last demon standing becomes the new king. The story follows Gash Bell and his human partner, Takamine Kiyomaro, as they navigate this intense and dangerous competition.