The Psi destroyer is a piece of terran technology developed by the Dominion that had the ability to generate an energy field that harmed zerg that had a connection to their psychic hivemind, disrupting their psionic link and tearing them apart on the cellular level. This large machine was fed by numerous generators which created a large field to encompass the battlefield.
At least one of these devices was constructed on Korhal by the time the Zerg Swarm arrived and invaded the planet. Due to its effects, the Queen of Blades was unable to deploy her troops as they were wounded by the machine's power. However, in her entourage was Dehaka and his pack of primal zerg who lacked the psionic hivemind connection. Thus, he and his forces were immune to the Psi destroyer's power leading them to be deployed to destroy the power links feeding the machine.
After each power link's loss, the Dominion forces attempted to reroute the power but ultimately Dehaka destroyed all the links. This allowed the zerg to attack the Psi destroyer itself which allowed Kerrigan to stage her final assault where she intended to kill Emperor Arcturus Mengsk.
The following section contains information from a previous version of StarCraft II: Heart of the Swarm which is no longer valid.