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In the depths of Ulnar, Amon opened a portal to the Void that sent a cascade of destruction Void energy over the area. The portal drew energy from the Void crystals in the area to remain open. The Daelaam destroyed them, and the portal was closed.
There are four void crystals around the map. The player must destroy them to win the mission. The garrison defending them varies.
The crystal is labeled as an "Ulnar teleporter" in spotlighted game assets.