Wraithknight
The Wraithknight is a massive Wraithbone construct, towering even over the tall Wraithguard and massive Wraithlord design to harvest new soul stones from Crone Worlds. They can be armed with a pair of massive fuck-off Wraithcannons, a Suncannon or a Scattershield Duel Disk and a huge Ghost Glaive.
They are for the Eldar what the XV-104 Riptide and Dreadknight are to the Tau and Grey Knights, except less stupid in the case of the latter. It is an enormous model and a lethal one, making it a massive fire magnet in a more literal way than usual. While it's WS and BS are kind of meh (both 4), it's a Jump Monstrous Creature with S 10 and 5 attacks and I 5. It's impressive in defence but not too much: T8, 6 wounds and 3+ armour; also the option of a 5++ blinding anyone at 6" each time it saves this way. Cool, but sort of disappointing. Deepstrike it for that O.O look on the other guys face.
Like other wraithbone constructs, Wraithknights are animated by souls of dead Eldar. Wraithknights are special because they also have pilots; in particular, the pilot must be the twin of the soul animating the machine. It seems that the designers must have been watching Evangelion or Pacific Rim the night before designing the Wraithknight.
The Wraithknight is usually regarded as less unfair than the Ion Accelerator Riptide due to not having a 2+ save, having a worse invulnerable save (when it gets one at all), having Jump instead of Jetpack which is superior for ranged creatures, not having access to FNP, being more fairly costed, and having less impressive weapons. That being said, the Wraithknight has a superior statline in assault compared to a Tyranid Scythed Hierodule; which costs twice as many points and is a Lord of War. This is hilarious. Yes a barbed Heirodule's bio-cannons will make for un-fun times, but if you can get into assault with it; you've probably crippled the Tyranid's much more expensive unit.
The weapons the wraithknight has include the Sun cannon, a plasma executioner that doesn't get's hot in exchange for only being S6 instead of S7 attatched to a more accurate (and arguably more durable vs many weapons) platform; and is thus mostly used to make high armor save units cry while still having a dinky invulnerable save. The Wraithcannon, which is an S10 AP2 Distortion weapon with decent range, making for good vehicle poppers and if you get a six to wound on someone's expensive character or monstrous creature you'll be laughing all the way to the bank. And of course, the Ghostglaive which is completely pointless as you're already S10 and master-crafting isn't that special.
On their shoulders, they can put most of the same sets of weapons you can put on special weapons platforms or Grav-Falcon turrets. They will usually be pointless as you can only fire two weapons at a time.
Wraithknights like to be fortuned to forever laugh at all small arms fire and turn a one in three chance to successfully invulnerable save to a one in half chance, concealed, and renewed, but sadly cannot have their armor saves enhanced. Plonk one on a skyshield and place fortune on it to sneer at the very concept of dying. Don't forget that Forewarning and potentially Sanctuary are options, too.
As of the Codex Eldar: Craftworlds update, Wraithknights are now Gargantuan Creatures (cue outrage from the few remaining Nid players), and the Wraithcannon and Ghostglaive are now Strength D. Eldar players celebrated, while everyone else is wondering what the hell GeeDubs was smoking to think that buffing an already overpowered unit further still could possibly be a good idea. On the bright side, there are heavy hints this thing is now LoW, so in a Battleforged army you get a maximum of 1 in your list. However, it's not a merciful note since literally this is now the rape machine to end all rape machines.