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, darkness extinguishes light 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... At least until the story advanced with 8th edition. They were instrumental in slamming the door shut hard into Slaanesh's face when he/she/it tried to invite itself into Commoragh while Asdrubael Vect and Lady Malys were too busy with their powerplays after the whole Ynnari thing. They even killed Vect. However he ended up being brought back to life at his own funeral, and it's unknown whether Vect hired the Mandrakes to "kill" him or someone else got the drop on Vect and Vect's funerary resurrection was Not As Planned for them.
In Older editions, they were basically just Dark Eldar dressed in BDSM leather and covered with knives; this style is seen in Dawn of War Soulstorm. In current edition, seen to the right, they look like shonen villains, fitted with pants worn by Japanese Kendo practitioner.
They apparently eat daemons now too.
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 by giving them much needed staying power and a ranged attack, making them actually pretty points-effective backline objective campers, and no longer made it necessary to play them as Harlequins in order to merely get them onto the table.
Then 8th Edition happened and they became badasses in their own right, so there goes that meme for good. Maybe saving Commorragh and kicking all the invading daemons out gave them a confidence boost?
|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 Fiends - Khymerae - Razorwing Flocks - Ur-Ghuls|
|Pain Engines:||Cronos Parasite Engines - Talos Pain Engines|
|Vehicles:|| Raiders - Raven Fighters - Razorwing Jetfighters |
Reapers - Reaver Jetbikes - Venoms
|Super Heavies:||Ravagers - Tantali - Voidraven Bombers|