"To those who have turned their faces away. To those who are faithless and wretched in their jealousies.
To those who have denied us. To those who have denied me.
I will wreak vengeance. I will wrench your souls and break your bones.
I will cast hunger through your accursed existence. Down the eons, you will not forget.
I will grant you this gift from love turned aside and make you like me, break you in my image as you have broken me.
I shall cast the fear of myself into you and all of your kind.
I am Llandu’gor. I am the hunger.
I am the flayer, and from this moment, you shall be too."
- – The Curse of Llandu'gor the Flayer
"They'll rape us to death, eat our flesh, and sew our skins into their clothing. And if we're very, very lucky, they'll do it in that order."
- – Corporal Zoe Washburne, 57th Overlanders Brigade
"Some hundreds of years may have passed. I don't know. AM has been having fun for some time, accelerating and retarding my time sense. I will say the word now. Now. It took me ten months to say now. I don't know. I think it has been some hundreds of years."
- – Ted, I have No Mouth and I Must Scream
Flayed Ones are Necrons who go about wearing the bloody flesh of their victims. They are victims of the Flayer virus, a program error where they develop a desire to reclaim their flesh and blood existence by devouring the flesh of the living (And since they don't have any kind of digestive system, the eaten flesh just falls down through their ribcages).
It's the end result of the C'Tan known as Llandu'gor the Flayer, who was not sharded like the others, but completely destroyed. Still, he had enough warning to curse the Necrons who killed him to suffer the Flayer virus as a last "fuck you;" they spread the virus to others, who spread it still further, and so on for quite some time until the other Necrons caught on.
Flayed Ones are basically the Lepers of Necron society, feared and ostracized for their disease. Although instead of sending them to a Leper colony, the Necrons simply try to kill them to prevent the spread of infection before it fully emerges (that's disease control in Warhammer 40,000 for you); the ones that don't die (and being Necrons with the ability to self-repair even after being nuked that's most of them) end up in a weird subspace dimension where they hang out with other Flayed Ones until they sense bloody battle, at which time they warp out, kill everything with a pulse, and warp back there again once they're done. The "clean" Necrons don't mind this so much as long as the Flayed Ones fuck off as soon as the battle is up.
This new lore comes from the Fifth Edition Necron Codex. In Third Edition, Flayed Ones were just close-combat specialists who liked to play dress-up with the skins of their slain foes. Their choice of attire was because they'd gone insane, either from being roboticized or because the hibernation protocol failed and they were forced to spend 65 million years awake and immobile I-Have-No-Mouth-And-I-Must-Scream-style.
The novels/stories have added new dimensions that make them even nastier. In one book, the Flayed Ones flay people because in their warped minds they're just trying to turn themselves back into beings of flesh and blood. In the Psychic Awakening story "Gone Dark" their intelligence and sadism get a boost; a Flayed One flays a guardswoman and wears her skin before messing with her brother by impersonating her - including flawless voice mimicry and awareness of their relation - to get close to him and then attacking.
Unfortunately, their models and their lore were not updated well -- they went from "pray we kill you before we flay you" to "dance party."
That is, until 9th edition hit and now they have some great plastic models which resemble those from Dark Crusade.
The Bone Kingdom of Drazak
A nice piece of story from the 5th Edition Necron Codex tells of a legendary Tomb World named Drazak (located in the Ghoul Stars) or alternatively the Bone Kingdom. It's a world populated almost entirely by Flayed Ones, who are ruled by Valgul the Fallen, a Necron Lord who is immune to the Flayer virus. The world is devoid of living creatures, so the Flayed ones roam around fighting each other for scraps of rotten meat and bone. Every few months Valgul rises from his throne of bone and announces a Time of Bounty, where they launch fleets to raid nearby planets to collect flesh and blood of the living to sate his subject's hunger. Which leaves the questions a) Why anyone would stay on those nearby planets, b) Why nobody has performed Exterminatus upon Drazak yet and c) Why the Flayed Ones return to Drazak instead of crusading across the stars.
All three questions are answered by one word: Grimdark.
On the tabletop
They used to
mostly suck. They were, and still are, Elites (one of the most competitive slots in the Necron army), they have one of the few 4+ armor saves in an army of MEQs, they could only do close combat but only have Initiative 2, they can't take transports, and they couldn't score. What they could achieve was based on their cheap price (same price as Necron Warriors), large squad size and their ability to Deep Strike and Infiltrate; basically, you fielded them along with Warriors and Scarab Swarms if you wanted to play Necrons horde-style...
But never again will the Flayed Ones be one of the prime examples of bad design - the 7th Edition Codex is arriving, and it heralds a new time for our favourite robot lepers. While their statline remain unchanged, they now count as having two separate melee weapons, for an extra attack at no extra cost. This of course is pretty good... But it doesn't stop there. Oh no. Not only were the Flayer Claws made separate weapons, they gained Shred and AP 5 as well. Fuck yes. Throw ten Flayed Ones into some Guardsmen and unleash 40 Bolter hits with Shred (mmmm guardsmen my favorite). Remind your opponent to bring a handbroom to remove his mutilated hordes easily.
Maynarkh Dynasty/"Dark Harvest" supplement from Imperial Armor also fixed most of their problems their own way. They can be Troops so as not to fight for an Elites slot, being Troops gain Objective Secured, and they can buy Flensing Scarabs to re-roll failed to-wound rolls for a turn.
They're okay. They're still an elites choice (and cost almost as much per model as Praetorians) but with their deep strike and +1 to morale tests for nearby enemy units they can be very useful for probing the enemy back lines. Unfortunately with the deep strike rules they're gonna need a 9" charge to get into combat on the same turn they deep strike and have only melee attacks, but if they do get into combat they can seriously fuck up MEQ and below. A full squad of FO can deal out a face-melting 40 attacks per turn with a re-roll to wound. Take them with Anrakyr to deal out 50 attacks and watch your opponent cry as you delete his units. Still relying on making that 9" charge though (although you can reduce it to 8" if you use Anrakyr's MWBD on the flayed ones, also meaning you'll get 50 attacks hitting on 2s).
- Never Forget You, a sad story about a Flayed One.