The Dark Templar cannot cast spells. Unlike their lighter counterparts, they do have a melee attack of 45 damage. However, they gain the additional advantage of being cloaked and a pair can merge into a Dark Archon. The Dark Archon is the Protoss's new spell-wizard. Its abilities include Feedback, Maelstrom, and Mind Control.
Feedback removes a unit's energy and does damage to that unit equal to the amount of energy removed.
This spell does not work on buildings. Maelstrom stuns all biological units making them immobile and defenseless for a short amount of time. It can also be used as a cloaking detector, however, it is useless against burrowed units like the Lurker. The big kicker is Mind Control. A Dark Archon can take control of ANY unit.