Though the dark archon deals less damage than the normal archon, it still deals a lot of damage and deals splash damage to clumped enemies. Its abilities are more supportive than the high templar and ascendant, making the dark archon a unit that demands less micromanagement in return for being less effective offensively without the psionic skills of the others.
An important note when using the dark archon's mind control is that in the single-player campaign, Mind Controlled units do not count towards supply, and using Mind Control no longer drains the dark archon's shields. If the player takes control of an SCV or drone, they cannot build structures, but SCVs can Repair the player's protoss mechanical units and structures. Heroic units, including all hybrid, are immune to Mind Control. Ultralisks have the frenzy passive ability, which makes them immune to Mind Control. Motherships and Shadows of the Void are also immune to Mind Control.
Mind controlled units have the dark archon's quotes rather than their own quotes.
Some artworks suggest that dark archons were used by Tal'darim, but they do not appear in the game. Data in the game suggests that originally, Tal'darim blood hunters would merge into dark archons. In the final product, normal Tal'darim archons use the dark archon model.