The Crystal is a powerfull and magical rare material, that Hecate uses to absorb Hades' powers, so that she will become the new Goddess of the Underworld. It is used in the episode, "Hercules and the Underworld Takeover", from the televiasion series, "Hercules", and also in some of the villains tournaments. When the Crystal drains all of the powers of Hades and Hercules steps in the Underworld to break the Crystal, it takes a form of a personality, as it transforms into a giant monster.
Disney Vs Non Disney Villains War - Part Two
In the battle of Hecate and Devimon against the Skeleton King and Queen La, one of the jewels resembles the Crystal, that Hecate takes at the end of the fight. It is possibly that it is indeed the Crystal, since Hecate uses the Giant Crystalline Monster, a monster resulted by the Crystal's powers, to fight off Nergal and Count Marzo. Another speculation, is that the Crystalline Monster is a part of the Crystal of Darkness, Chernabog's source and strenght.
Disney Villains War
The Crystal makes a minor appearance in the first war. When Hecate is confronted by Ayam Aghoul, she uses the Crystal to her advantage to blow Aghoul away, although this attempt fails, as it didn't kill the sorcerer, directly.
Disney Villains War 2
During the events of the second war, instead of Hecate, Mirage uses the Crystal to absorb Hades influence in the Underworld, so that she would become the new queen of the Underworld. When the Sorcerer's Society strikes in the Underworld, Mirage uses the Crystal, transforming it into the Crystalline Monster. She then sends it to kill the sorcerers. While the Crystal Monster manages to take out Yzma, it is no match against Madam Mim, who transforms into a rhino and then destroys the Crystal Monster. Despite the Crystal's destruction, the source of it, the powers of Hades, remains at the hands of Mirage. The sorcererss manages to rebuilt another Crystal, that it would keep the powers of Hades within and prevent anyone to reach it. The Underworld, later, gets attacked by Doctor Facilier, Hoagy and Queen Narissa, When Mirage gets enraged and blast through lightning Narissa, she mistakely breaks the Crystal. Destroyed, the source of monster, the flaming orb, goes to it's original being, Hades, much to Mirage's fury.
Disney Vs Marvel Villains War - Part Three
The Crystal appears in the third part of the war, as is used by Hecate to absorb Hela's lifesource, reducing her to a pile of ashes. It is not seen afterwards, due to the series discontinuity.