Ultimate power, a bit of madness, the risk of dying horribly... what's not to love about this.
I like the flavor that bleeds through. These aren't normal sorcerers who gather power through bound spirits. There is an entire ecosystem of ethereal creatures. It speaks to a larger, well-developed world, with enough similarities to standard fantasy to make it easy to grasp and enough uniqueness to set it apart.
This is good. While the concept may not be new, the backstory really brought it to life for me.
I would probably put these in an area where the previous owners (having lived) discarded them. Then, I would herd the players to this spot, without weapons of their own (perhaps they were captured and are attempting to escape.) Ok, this sub brings out the evil GM in me, but I like to keep my players on their toes.
These would also be a good tool for conmen. Sell them to unsuspecting adventurers, impressing them with the cutting ability, then set upon them on the road, knowing that they would be hampered. Steal from the corpses, and repeat.