The Mystical Orb (originally called the Giant Ball of Death) is a weapon used by Moloch the Oni Destroyer, which resembles a stone sphere with Chains wrapped around Moloch's Arm, this ball also has strange line markings on it. It first appeared, along with him, in Mortal Kombat: Deadly Alliance then in Mortal Kombat Armageddon.
This cursed "treasure" can be seen in a concept art sketch when it was carried off by an owl named Herald before being returned to the hands of the Oni Moloch. It is the second last relic to collect in Konquest, being found in the Edenian Ruins.
In Mortal Kombat: Armageddon, it doesn't seem Moloch possesses it, but it appears during his Versus Render, and during some of his special moves. It was featured as one of the collectible Relics under the name "Moloch's Ball" found within the game's Konquest mode as well.