The Apple of Discord is Eris' item of power, an apple made of gold and causes all sorts of chaos, discord and strife. Billy refers to the Apple as a "Magic Banana", although in Creating Chaos, he saw the apple and was able to identify it correctly. In Complete and Utter Chaos it broke into 3 parts. Billy, Mandy, and Grim messed with it, and fused together into a cow-like monster with Billy as part of the udders, Grim has control of the legs, and Mandy's hair turned into crab-claws.
After the events of Underfist: Halloween Bash, in what was supposed to be an episode of Underfist, the Apple split Eris into two entities: Nice and Naughty Eris. They may have come to take revenge on Hoss for breaking up with her and may have tried to get Fred Fredburger to work for them by giving him apples of gold.