Wracks are what happens when you have a bundle of bodies stitched together like Frankenstein. In probably the most squick-tastic way ever imaginable, becoming a Wrack is mostly voluntary which means that they are Dark Eldar who decide to dedicate themselves to serving a Haemonculus as a servant. Essentially some would say that the Wracks are the literal pinnacle of Dark Eldar BDSM. Reasons include wanting the protection of a Haemonculi Coven rather than a Kabal, trying something new, hiding after angering an Archon or simply being bored with life and wanting something to 'spice it up.'
The Haemonculus modifies their bodies into something that serves them better ("them" being the Haemonculus) such as being injected with all kinds of steroids and drugs, cut up into pieces and re-assembled. Their arms usually get replaced with various weapons and tools which can be read about below. Once the procedure is mostly complete, they then put a metal faceplate on them as personal identity has no place in a Wrack's existence (supposedly; a Wrack in a novel named Xagor mentioned his brother and seemed to have a life outside the Coven). They act as assistants and bodyguards, Wracks and their masters often compete with Tech Priests and their Servitors in a friendly game of "whose personal lab assistants can survive the most abuse possible?" Usually the most likely outcome would be the Haemonculus and Tech Priest start punching and smack talking each other on whose cyber-technology or bio-technology is the more efficient form of progress...which is something many in /tg/ wish would happen.
But back on topic, Wracks are equipped with a range of cruel weapons including razor-sharp blades, needles, poisons and even more exotic and disturbing weaponry. This list of weaponry includes but are not limited to, the feared Liquifier guns, Venom Blades, Flesh Gauntlets, Ossefactors and Mindphase Gauntlets.
Amongst the Wracks there is a position known as Acothyst. This seems to be a position of authority amongst the Wracks, perhaps equivalent to a captain or commander. These Acothysts are sometimes rewarded with unique Haemonculi weapons such as the Hexrifle which is a gun that fires the 'Glass Plague' which somehow turns its victims into glass statues... the only possible explanation without declaring it full on space magic is that the plague is a virus that converts biological matter into silicate crystals (it's been mention in other series, it's possible). Of course this is just /tg/ getting shit done, not like GeeDubs is gonna confirm it anyway. Edit: The new Fabius Bile book, Manflayer, seems to confirm this.
Interesting side note; calling them "Wracks" originated from the bony spikes emerging form their body making them literal "tool racks" for the Haemonculi, as well as the word "wrack" as in "wrack with pain".
|Forces of the Dark Eldar|
|Command:||Archons - Haemonculi - Kabalite Trueborn |
Lhamaeans - Medusae - Sslyth - Succubi
|Troops:||Beastmasters - Commorragh Slaves - Grotesques |
Hellions - Incubi - Kabalite Warriors - Mandrakes
Scourges - Wracks - Wyches
|Beasts:||Clawed Fiend - Khymerae - Razorwing Flock - Ur-Ghul|
|Pain Engines:||Cronos Parasite Engines - Talos Pain Engines|
|Vehicles:||Raider - Ravager - Reaper - Reaver Jetbike - Venom|
|Flyers:||Raven Fighter - Razorwing Jetfighter |
Raptor Fighter - Voidraven Bomber
|Spacecraft:||Impaler Assault Module - Slavebringer Assault Boat|