The Dark Archon is a Protoss spell-caster that does not have the ability to attack directly. Rather, it is a supporting Protoss unit that is capable of three powerful spells. It is formed by merging two Dark Templars, which are produced by the Gateway.
Because of its high cost, both in terms of minerals/Vespene Gas as well as the loss of two powerful Dark Templars, and its limited abilities, they are not commonly produced during competitive play. Rather, they are produced for relatively specific applications in the late game.
Occasionally, Dark Archons with a fast Maelstrom can be used as a counter to Zerg Mutalisk Harass. This freezes Mutalisks in place while Psionic Storms, Archons, or Corsairs deals damage to them. To maintain mobility, as well as to keep the vulnerable Dark Archon protected, a Dark Archon and Archon can be stored in a Shuttle to quickly reach the Mutalisk stack, followed by with Maelstrom.
Dark Archons are also occasionally seen in long PvZ matches. Supported and defended by a combination of Archons, High Templars, Reavers, and Zealots, the Dark Archon is safe to cast its spells against late game Zerg armies. Maelstrom can effectively freeze packs of concentrated and fast-moving Zerg units such as Ultralisks, Zerglings, and Overlords preparing for drop-play while dealing them damage with Psionic Storms and other Protoss units.
Mind Control isn't used as extensively as the other two spells, but can serve some utility in taking control of high value Zerg units such as Ultralisks or Defilers. It is noted that any Mind Controlled Overlords will result in the Protoss player controlling any Zerg units transported by the Overlord.
For an extensive discussion on Dark Archon use in the modern PvZ match-up, see this Teamliquid thread.
Dark Archons are usually not used in the PvT match-up as their spells are not particularly useful against most common Terran strategies.
Maelstrom is ineffective against mechanical units. Mind Control, however, can be used should the Terran opponent elect to produce Battlecruisers.
It is noted the Dark Archon is part of the rarely played Royal Stove strategy, which is used to Mind Control an SCV and eventually tech to Factories so that the Protoss player can recall Sieged Tanks.
Although rarely seen in the PvP match-up, Dark Archons may use Feedback to counter the opponent's High Templars. Because the Feedback spell can be cast at a very long range, it can be used to snipe High Templars, such that the High Templar's own energy is used against itself.
In other applications, Mind Control can be used to steal away units of high value such as Carriers or Arbiters. It is noted that any Mind Controlled opponent Shuttles will result in the opponent losing control of any units transported by the Shuttle.
- Stork vs GGPlay 2008-06-18, Shinhan08 Proleague Extensive use of Maelstrom and Mind Control.
- Violet vs HoGiL 2009-06-06 Shinhan 08-09 Proleague Violet uses a single Dark Archon with Maelstrom against hive tech Zerg at about 43:45.
- Violet vs Hyuk 2009-11-09 Shinhan 09-10 Proleague Violet's single Maelstrom solves the problematic High Templar sniping.
- Movie vs Shine 2010-01-08 Ever 2009 Starleague Movie gets a fast Maelstrom to negate Mutalisk Harass.