From Lexicanum

A Mandrake is a type of Dark Eldar whose physique has adapted to make him camouflaged almost anywhere. Advanced infiltrators, Dark Eldar Mandrakes strike from the shadows, surprising and rending their enemies apart. They have shadow-skin, which is possibly from warp exposure, that makes them incredibly hard to see when combined with their stealth. Mandrakes are often cannibalistic and wild, living separate from other Dark Eldar.

Sources

• Codex: Dark Eldar

Dark Eldar Army List
HQ	Archite • Archon • Dracite • Dracon • Haemonculi • Incubus
Elites	Grotesque • Mandrake • Succubus • Warp Beast Pack • Wych
Troops	Raider • Sybarite • Warrior
Fast Attack	Hellion • Reaver Jetbike
Heavy Support	Ravager • Scourge • Talos
Special Characters	Asdrubael Vect • Decapitator • Drazhar • Kruellagh the Vile • Lelith Hesperax • Urien Rakarth