The dark templar is a cheap hero obtainable in early game and at the start if fast mode is selected. the dark templar has a small cleave, very high melee damage, and is always cloaked. the dark templar also has the ability to blink decent distances to escape harm or scans.
the dark templar ( also known as DT ) is more effective in early game play, as a very powerful defensive hero and a very powerufl ambusher hero. the DT makes a poor bass attacking hero due to Turret detection and higher risk of scans.
the DT is best used early to pick off any marines destroying rocks, or and mercs going for early attacks, but as stated earlier the DT makes a great defensive unit in its ability to kill a large chunk of your opponents force before even getting to your base. becareful around concussive shell marauders, one scan and your DT may be done for.
the DT can also be combine with another DT or a high templar to form the all powerful Archon!