The Darkstar Fighter is a little known Eldar aerospace fighter found only in Battlefleet Gothic.
Darkstar Fighters are Eldar fighters designed specifically for space (Although they are just as capable of doing maneuvers in the air in contrast to the Imperium's Starhawk Bomber and Fury Interceptor). They are highly advanced, using technology to provide them with great agility and speed, using some form of inertia-dampener which allows them to turn on the spot. They run on crystal power generators which allows them to operate for longer periods than many other more conventional engine forms used by other races.
Battlefleet Gothic Armada
In Battlefleet Gothic: Armada, the Darkstar Fighter is the Elfdar analogue for the Imperial Navy's Fury Interceptors which is not that surprising really. The prime job of an Interceptor is to destroy any incoming torpedoes and protect the ship from enemy bombers. Darkstar Fighter usually fly close to their parent ship in an effort to repel any potential boarding craft if the point defense from the ship proves either ineffective or destroyed.
Darkstar Fighers are considered one of the best fighters in the game due to, you know, because they are Eldar. Being able to hit harder and faster than many of the lesser races fighters save for the Necrons and their Edgier Kin.