Dark Archons are two Dark Templar's who have sacrificed themselves to transform and summon a Dark Archon. After a Dark Archon is created, it loses the ability to attack and is generally left behind. However, Dark Archon have two great abilities that allow them to dominate. Feedback can only target units with energy, which can destroy other protoss and rid the fear of a Nuclear Strike from a Ghost. But their most prominent attack would be called Mind Control which can be put to good use when you're near out of units and building yourself.
Dark Archons are not introduced until Brood War, leaving a while until you can enjoy the taste of destroying your opponents with their own race. During Brood War, they are introduced in the first campaign after returning from retrieving the two crystals Khalis and Uraj with Sarah Kerrigan. Aldaris will pull together the protoss refugee's from Aiur and turn on the Dark Templar capital. Zeratul will introduce them as you start the fight, which allows you to, instead of killing the protoss refugee's, Mind Control them to put them back on your side.
 Game Usage
 Protoss vs Terran
Dark Archons are rarely used against Terran. However, they can be used to Feedback Science Vessels to prevent EMP Shockwaves.
 Protoss vs Zerg
A late-game Protoss army may consist of a few Dark Archons. Feedback is used to kill Defilers from a distance and Maelstrom can freeze all Zerg units to make them vulnerable to Psionic Storm.