The Unfleshed were a faction of mutants that were the results of rejected Chaos Space Marines "recruits" that were formed in the Eye of Terror thanks to a few sick fucks. They were formed using the Daemonculaba, which used human women who were physically bloated and twisted and then forcibly impregnated with adolescent human children with a reverse c-section, along with stolen or scavenged geneseed. Many of these kids were literally reborn as Chaos Space Marines, though most ended up dead or catastrophically mutated.
All of these kids are reborn with no skin upon them, and if they pass the Warsmith's standards for murder machines, they get new skin and a set of power armour. If they don't pass the mark (such as coming out dead or as a horrible abomination), they're quite literally flushed into the sewers, and the few that survive are forced to live on without skin. The Unfleshed that survive are XBAWKS HUEG and humanoid (at best) with muscles that are as hard as iron and completely dulled to pain; and they are tough and numerous enough they've been able to build up a sizable population. They are also cannibals, and love those meaty little humiez. (Or are they? Cannibalism is eating beings of your own species, and I'm unsure Unfleshed can be called humans anymore...)
They also worship the God-Emperor of Mankind, and even asked Ultramarines Captain Uriel Ventris for His forgiveness for being abominations, and they have a child-like mind set since they are literal (wo)man-children. In a way, it makes one feel kinda sorry because they are literally unwanted abominations that were forcibly brought into this world, and they just want to die.
They help Uriel, his buddy Pasanius, and their posse of outcast Space Marine guerrillas to take some revenge on the Iron Warriors and to do the Emperor's good work. They end up having to flee on a daemon engine to the world of Salinas. Uriel promises them a home somewhere in the Imperium, but he gets caught up in the local political struggle between the local insurgents (loyalists-turned-freedom-fighters) and the Imperial authorities (Imperial-Guard-turned-irredeemable-assholes). The former insurgent leader turns out to be a vengeful psyker, and is able to mind control the Unfleshed due to their (un)natural affinity with the Warp and the presence of a bunch of angry ghosts due to a recent massacre carried out upon the insurgents. The Grey Knights step in, and with their backing Uriel has to go pull an Old Yeller on the Unfleshed. The main Unfleshed character and leader (known only as the Lord of the Unfleshed) manages to get a "happy" final moment by seeing his reflection as it would be if he wasn't a skinless abomination.