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The particle cannon is an anti-air weapon used by the Tal'darim of Atrias. A number of these weapons were mounted on the ramparts of the Temple of Erris. While Zeratul infiltrated the temple, Talis asked him to destroy these particle cannons so she could send in reinforcements.
Three particle cannons appear in the mission "Evil Awoken," each guarded by enemy forces. Each time a cannon is destroyed, a warp prism delivers additional units to the player. The particle cannon is constantly firing, but this is cosmetic, and is otherwise a background structure.