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. Most Dark Eldar think they're just useless scumbags, too primitive to be worth paying attention to, though some archons hire Mandrakes as assassins, as they're quite good at it. 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 Commorragh with a little help from Nurgle, but Vect fucked them so hard only a handful of their population survived, and since then they stayed low.

Unfortunately, despite their superpowers in fluff, on the tabletop Mandrakes were notorious as one of the WORST infiltrators of the game, and stood alongside the Pyrovore as one of the worst units in the game, period. In an army so reliant on hit and run tactics, Mandrakes literally had one job, and it was the one job they couldn't do, but the models looked awesome and it became something of a running joke with Dark Eldar players as to what their Mandrake models counted as. Then came 7th Edition, which did much to alleviate their uselessness, and making it no longer necessary to play them as Harlequins in order to get them onto the table. And then 8th Edition happened and they became badasses so there goes that meme. Maybe saving Commorragh and kicking all the invading daemons out gave them a confidence boost?