In Games Workshop's tabletop wargaming setting Warhammer 40,000, the Dark Eldar are depicted as a faction of sadistic, piratical raiders with an affinity for torture and cruelty. They are a playable faction in Warhammer 40,000, supported by rules supplements and a miniatures range, and also appeared as a playable race in the video game Warhammer 40,000 Dawn of War - Soulstorm. They are closely related to the other Eldar factions in the fiction; the Craftworld Eldar, the Harlequins and the Exodites, and share several of their characteristics. They also bear some stylistic similarities to the Dark Elves of the Warhammer Fantasy Battles setting.
In the context of the Warhammer 40,000 tabletop game, the Dark Eldar are an exceptionally rapid, mobile army. They make use of anti-grav skimmer vehicles to transport their units around the tabletop without behind hindered by intervening terrain, and these vehicles are some of the fastest of their class in the game. Dark Eldar units typically have much higher initiative statistics than the majority of other Warhammer 40,000 armies, allowing them to attack first in the close-combat phases of the game. They also make extensive use of poisoned weapons, a game mechanic which allows the wielder to damage models with high toughness characteristics just as easily as they would a much weaker opponent.
There are some trade offs for these strengths; Dark Eldar units themselves are relatively weak and lightly armoured, succumbing to damage more easily than many other forces. The Dark Eldar rely on removing enemy models with their quickness of attacks before their opponents have a chance to react, and the use of cover to help protect their lightly armoured troops.
The Dark Eldar are one of the few armies in the game not to make use of their own Psykers, but they do have a few options in their Codex which can disrupt enemy psykers. It is also possible for them to bring psykers to the battle by allying with other factions; the Dark Eldar are considered "Battle Brothers" with models from the Eldar and Harlequin factions in the game, allowing units from these different forces to fight alongside each other without penalty.