This is a weapon that has been used by long gone barbarians. They were called Hauls (some old texts write them Gauls, as their alphabet did not contain an "H").
The sackbutt developed its full power over the course of a battle. It was a musical horn of a kind attached with a sack to one end. When fighting the Hauls would start with a sword as most armies do. But every time they severed a limb (or part of) they would stop to put it in the sack. When reaching a sufficient weight they would put away their swords and "butt" ( it´s called butt also due to the sound generated by a sack full of human parts hitting a living being) the enemy. The most fierce Hauls put even body parts of themselves into the sack, but this habit died rather fast...