The Heart of the Visorak was an artifact that could summon the Visorak horde from wherever they might be.1 It was a golden crystal about the size of Pohatu's head, inexplicably suspended in mid-air. When the Heart touched the ground, it summoned the Visorak from all ends of the world to its location. This explained how the Makuta assembled the horde.2
Notes
The Heart of the Visorak was an artifact that could be used to summon the Visorak hordes from anywhere in the universe.3
The Heart of the Visorak appeared as a golden crystal. Wherever it was, the Visorak would come, traveling from everywhere in the known universe to find it. [Johmak] said that it was currently active and that its power was growing. Somehow, Takadox had heard of the Heart of the Visorak, though never seen it.4