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Several null circuits were deployed on Cybros to provide the power needed to keep the Purifiers in stasis. When the Daelaam came to Cybros to release the Purifiers, they destroyed the null circuits to do so.
There are twelve null circuits around the map in the mission, three in each quadrant. Destroying three in the same quadrant releases a force of Purifier units who will clear the area of zerg. When all twelve null circuits are destroyed, the mission is won.