pillow-case full of soap

Another variation of this is the "Bar of soap in the sock". This is equally if not more deadly as it is less unwieldy and lighter. People have died this way from ribs puncturing internal organs and severe internal damage/bleeding.

In the US Navy, this was called a Blanket Party. When I was in boot camp, this was a constant threat given out by the recruit company commander. One actually happened, and it happened to the guy sleeping in the rack above me. Four guys grabbed the corners of his blanket, and two guys thwacked him repeatedly with a bar of soap in a sock. The guy had caused us to fail an inspection (and no matter what, something would've caused us to fail the inspection. We were supposed to fail it, and there was no way for us to pass. It had something to do with learning discipline and not getting pissed at those above us.)

Unfortunately for the guy who was sleeping above me, he was "it". They would've beat on him some more if I didn't stop them (it helped that I was one of the oldest and biggest guys in the company). He ended up with big welts on his back.

If you're interested in what it looks like, there's a very accurate scene in Full Metal Jacket, a Kubrick film.