You may think an undead bone is no different from a normal bone, and in most cases that’s true. The animating forces of necromancy normally just puppet the bones, allowing them to be possessed by a spirit or controlled by some external force.
The undead of Fellhollow are different. They animated by the last remnants of their humanity, tiny bits of soul clinging on in tattered forms. They move because they need to and they shudder because they fear what happens when they stop.
As such, the bones of the Fellhollow dead have strange properties that a skilled craftsman can refine and find uses for.
Morbid, yes, but inventive.