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- Protoss dark templar(src)
Stealth Close Combat Trooper
Dark templar are represented as permanently cloaked infantry with extremely powerful attacks.
Dark templar are permanently cloaked, like observers, and their effectiveness is heavily dependent on it. If the enemy has adequate detection capabilities it is more efficient to attack with predominantly zealots. Although zealots deal significantly less damage, they are more likely to survive the charge and last long enough to make a difference (as well as being much less expensive).
Dark templar could not be created in the original StarCraft. They were given to players as special units at the start of "Homeland" and "Eye of the Storm" (and acted as enemies in "Eye for an Eye"). They had slightly different stats from the Brood War version, and could not create dark archons.
In Brood War, dark templar were buildable units (statistics apply to Brood War units). They were frequently used in rushes against terran, in which the attacking player tried to destroy their opponent before they could develop adequate detection. They are sometimes fondly referred to as "dt"s.
Dark templar are both great harass units and great deterrents.
Dark templar can deter an attacker who doesn't have ready detection. In Protoss vs. Protoss, sniping the enemy's observers will force the enemy to retreat before even one dark templar. In Protoss vs. Zerg, keeping overlords away from the battlefield makes dark templar unstoppable. This naturally leads to the corsair/dark templar strategy, which dominated the pro leagues in 2007.
Dark templar are also fantastic harassers because they can kill probes or drones in one hit. Units that are killed in one hit do not send a "You are under attack!" warning to the opponent, which means a dark templar can frequently destroy a distracted opponent's economy without the opponent ever knowing what happened. Dark templar are thus one of the units protoss turns to get back in the game unless an opponent is prepared.
To get them into the opponent's base, shuttles are a frequent companion of dark templar. Shuttles can bypass frontal detection and base defenses and put dark templar right near the mineral lines, forcing the opponent to defend multiple fronts. It's not uncommon to see shuttles with even four dark templars or a few dark templar and zealots strike a base late in the game, hoping to eliminate the detection before help arrives.
Dark templar may be spotted by their visual distortion, and distinguished from observers by the unit's pathing, but detectors are needed to effectively attack them. Having a forge or engineering bay before dark templar appear allows photon cannons or missile turrets to be placed as required. It is difficult to build both the detector and the prerequisite building in time to meet a sudden appearance of dark templar.
Dark templar rely on stealth to compensate for having less health than zealots. Mixing dark templar with a larger force may mask their presence and attacks.