The Grim Adventures of Billy & Mandy

Grim's refusal to help Mandy win a karate tournament, and the $50,000 prize it promises to the winner, leads her to a curio shop, where the shopkeeper, Kuan Ti, gives her an ancient "Chicken Ball" which, when eaten, allows her to become a great fighter. Mandy easily wins the tournament and does not care about the money anymore, only about fighting. It turns out Kuan Ti is actually Eris in disguise, and this was just another of her schemes to create chaos. Seeing how he has a chance to get rid of the kids and finally be free, he tries to take advantage over this by making Billy a superb fighter too, so he and Mandy can fight ferociously to the death until the entire arena is destroyed. With both still alive, Mandy still demands the prize money, which she uses to buy Hector Con Carne's Bunny Island.