Torture is a bad idea in all worlds. But whereas in our world, torture comes back to haunt the torturer in mostly metaphoric (and photographic) ways, in fantasy worlds, the haunting is much more literal.
The shredskin is what remains when a tortured soul haunts its own flayed skin. Even after it has had its vengeance it still continually hunts for bodies to enshroud and control. It can also command undead—even intelligent undead like ghouls are not immune—driving them to attack PCs with a ferocity beyond even the usual undead hatred of the living.
Many shredskins are made of orc skins. This is no surprise—orcs love torture and flaying in equal measure, and are probably responsible for the accidental creation of the first shredskin. Some orc tribes even go out of their way to create shredskins, flaying hundreds of victims (often while subjecting them to bane or by using magic weapon on the knife doing the skinning) to spark the return of just one such skin. They then tether the shredskins like pennants to spears or ballista bolts that are fired into enemy camps. With so many bodies milling about, even tethered the undead usually has no problem finding a victim it can suffocate, possess, and use to cut itself free. In the confusion of war, whole platoons have been lost to a shredskin quietly moving from soldier to soldier.
Plague is rampant and bodies pile in the streets. The man pushing the corpse cart is in little better shape—he is a ghoul being commanded and ridden by a shredskin. The ghoul would have been happy to leave any adventurers they meet alone—there are corpses to feast on for weeks—but when one of the sellswords reminds the shredskin of its killer, it cannot help but attack.
Adventurers find a drum, the head of which moves and writhes as if alive. The drum skin is all that is left of a satyr flayed for insolence by a nephilim. If the adventurers free the skin, it will attack, hoping to possess one of them and hunt down the half-god. If they don’t and attempt to keep the drum, they will be attacked by every fey they encounter almost without exception.
—Pathfinder Bestiary 4 243
On Golarion, the Shredskin is also the name of an artifact made from the ribs and skin of the great Kazavon (see Artifacts & Legends). Now in the possession of Zon-Kuthon, the Prince of Pain, one surmises it could have inspired the creation of the undead creatures that bear the same name.