Mandrake

They are not to be confused with the Drow nor the actual "Scream until your head pops off" Mandrakes from a typical fantasy setting.

Mandrakes are creepy cousins of Dark Eldar, who look like coal-black elf-hobos, covered in rags and armed with all kinds of creepy sickle blades. They live in a sub-realm where light does not exist (like at all, lamps and fire don't work there), and have shadow-based superpowers. Dark Eldar think they're just useless scumbags, too primitive to be worth paying attention for, though some archons hire some mandrakes as assassins, given they're quite good at is, as their superpowers allow them to teleport from shadow to shadow, which is super-useful for sneaking through a cabal spire or haemonculi den, where every square meter is filled with traps, guards, monsters, fucked-up space-time anomalies or everything above.

Once upon a time they tried to conquer the Commorragh with a little help from Nurgle, but Vect fucked them so hard only a handful of their population survived, and since then they stay low.

Unfortunately despite their superpowers in fluff, on tabletop they are notorious as one of the WORST infiltrators of the game. And for an army that is reliant on hit and run tactics, the Mandrakes are completely pointless as they can't even do their job well...at least they aren't Pyrovores but they are still notoriously bad.