Unfleshed
The Unfleshed are a sort of rejected Chaos Space Marine that is formed in the Eye Of Terror thanks to a few sick fucks. They are formed using the Daemonculaba, which forcibly impregnates normal women with stolen gene-seed to create them. They are made with no skin upon them, hence their name, and since they don't pass the Warsmith's standards for murderous tanks of doom, they're forced to live without skin. Apparently, since they've been able to have a sizable enough population, it's possible that their muscles are as hard as iron or that they are completely dulled to pain. They are also cannibals.
Apparently, they are capable of believing in the God-Emperor of Mankind, as they asked Ultramarines Captain Uriel Ventris for His forgiveness for being abominations, and suggests that they have a child-like mind set since they are literal 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 do, but their leader gets a "happy" final moment seeing his refection as it would be if he wasn't a skinless abomination.