The Dark Templar is one of the best spy units the protoss have. The reason for this is because it's always cloaked. If you can disable all detectors in an enemy base your Dark Templars can attack with impunity. Two Dark Templars also have the option of turning into a Dark Archon with has the big plus of, with enough energy, taking over an enemy unit's mind and giving that unit to you. Beware because enemy units will atack that unit once taken over..
