The Crystal of the Magic Kingdom resembles a snowglobe and is one of the most powerful items in the Disney universe. Its power is able (and meant) to keep all things evil out of the Magic Kingdom park all on its own. When it's broken, the crystal itself becomes multiple shards, which are still able to have enough power to destroy any evil within.
Role in Sorcerers of the Magic Kingdom
Hades, who is bored with the nothingness that he has in the Underworld, is looking for a new summer home and begins to set his eyes on the Magic Kingdom. He plans on turning the park into a post-apocalyptic wasteland where he will rule. However, Hades recalls the Crystal of the Magic Kingdom's power to keep all things evil out of the park. The crystal was in the hands of Merlin, a powerful wizard who resides in a cottage in the middle of the forest. Hades sends his minions Pain and Panic to steal the crystal, but in a scuffle with Merlin, the crystal is accidentally broken into several pieces, with only four getting scattered across the park.
Hades, infuriated by the incompetence of his minions, then decides to recruit some villainous allies to help find the crystal pieces while Merlin recruits the park guests as sorcerers. A race begins to find the missing crystal shards and in during which, Hades and the park guests meet and interact with various Disney heroes and villains. Hades makes deals with the villains and bargains them to help him find the missing crystal pieces and, in return, will get what they want. (e.g. Cruella will claim all the Dalmatians in her possession; Yzma will become Empress of Adventureland; Scar will get immortality; etc.) However, some of the villains, like Ursula, Maleficent and Jafar secretly plot to double cross Hades and plan to use the crystal shards they find to rule the kingdom for themselves instead of giving them to Hades.
Eventually, the crystal is mostly restored, but the final piece is stolen by Pain and Panic and taken to the Underworld. Merlin and the park guests travel to that realm to foil Hades and his villain army, with Merlin trapping them in the crystal.
With the villains defeated and the crystal safe again, peace is restored to the kingdom and the guests become honorary sorcerers.