The Eldar needed a fancy jetfighter to allow them to gain air superiority and dogfighting prowess with all their usual stylishness. Enter the Hemlock Wraithfighter. This fast-flying vessel is manned by a Spiritseer assisted with spirit stones. This allows them to use some nifty psychic prediction to get the best targeting. This is good, because armed with two heavy D-Scythes, this thing can do swift hit and run attacks to wreck another army's shit.
As with other Wraith constructs, the Eldar tend to dislike using these forces unless absolutely necessary because the souls of Eldar dead are being used to power the thing, and that is looked upon as necromancy, but cleaner and a lot less direct. Also, the pilot apparently faces the risk of somehow having his soul transfer into the aircraft itself, leaving his body behind. So there's that, too. Finally, the main weapon of this vehicle works by literally separating enemies' souls from their bodies; while this is obviously very effective for the purpose of ending lives, the Eldar consider this technology unnatural and dislike using it.
An alternate form of aircraft that shares this body-type, the Nightshade, is used by the Crimson Hunters Aspect.
It looks like the experimental Su-47 Berkut. More like the old Monogram F-19A Specter model kit.
On the Tabletop
In 8th edition, the Hemlock is arguably the best standard Flyer Craftworlders have access to. A Psyker, this jet can know and cast one of the Runes of Battle (strictly the debuff side though) or Smite a nearby enemy once a turn in addition to a denial of one of your opponent's powers. This is a relatively unique role that can throw off unsuspecting opponents and gives it a decent niche as a frontline support unit for your other forces assaulting enemies on the ground. This is compounded by the Hemlock's Dropshock Pods, which drop all enemy units' leadership scores by -2 within a foot of the thing. This stacks with Horrify quite nicely.
This Psychic flexibility may make the craft seem lacking offensively, though this isn't quite the case. Heavy D-Scythes let your Hemlock open up 2D3 auto-hitting cans of S12 AP-4 2D rape on anything within 18" of it, letting it slaughter MEQ and TEQ targets wholesale with some good rolls. In many cases, this'll likely incur additional losses in enemy squads due to the aforementioned Dropshock Pods barring fearless auras or stratagem usage. The auto-hitting nature of the Heavy D-Scythes ironically makes the Hemlock a more effective anti-air vehicle than their Crimson Hunter kin, with the severe strength of the guns also serving as reliable anti-tank/monster firepower as well. Unfortunately, in the grand scheme of this infantry-favoring edition, having only 2D3 shots per turn (and a Smite if you elected to use it that turn) make the Hemlock ill-suited to directly dealing with Horde lists, though this can be mitigated somewhat if used as a component in leadership bombing runs (where it'll probably and hilariously cause more casualties from failed leadership tests than the kills themselves). Units like Shadow Spectres or Drukhari phantasm grenades can assist in capitalizing on this technique.