The Dark Orb is a magical artifact of unknown manufacture, and is essential for summoning and maintaining control of the Dark One. Only when the light of the moon (when it reaches a certain point in the sky) is focused upon the orb can it ultimately summon the Dark One.
Trogg used the orb to summon the Dark One so he could transform the captured Enchanter Kings into Berzerkers like himself. Unfortunately for the villains, Tails was able to steal the orb away when Trogg and the Dark One were preoccupied, and later pitched it off the Dimension Bridge over the non-existent space. When the orb ceased to exist, the Dark One vanished with it, his existence tied to the orb.