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When the united forces of the Daelaam, Zerg Swarm, and Terran Dominion entered the Void to confront Amon, they found him enveloped in a shroud of void energy that made him virtually invincible. The void crystals surrounding him projected this field. Their vulnerability was that they regularly needed to rise above the barrier to recover their energy. Despite fierce resistance from Amon and his minions, the allied forces destroyed enough crystals to disable the barrier, rendering Amon vulnerable. Sarah Kerrigan, having been transformed into a xel'naga by Ouros, then confronted and killed Amon.
Void crystals surface at various areas across the world map, their route displayed for the player. They are vulnerable to attack by both air and ground attacks units until they sink back into the void. Raynor or Artanis may send units to attack the void crystal, depending on which of their bases it is surfacing near.
When attacking a void crystal, they emit a field of void energy that deals damage over time to all nearby units, and lowers their move and attack rates by 20%. Attacking them causes void constructs to spawn near it on open ground. The player should move out in force and make liberal use of Kerrigan's abilities, particularly Celestial Radiance, to destroy the void crystals quickly without suffering crippling losses.
While the player must destroy seven void crystals to meet the mission objectives, there is an infinite number of them that will spawn, until the player has destroyed seven.