It has the vorpal weapon quality, but also retains the property of the final blade it was made from; anyone killed by it has their soul trapped and cannot be resurrected. The Gray Gardeners are able to free souls trapped by the final blades, so presumably they could free a soul trapped by the axe. The axe laughs when it kills.
The axe is currently in the possession of a frost giant called Bjarlsfarn the Bloody, who lives in the Fog Peaks and leads raids into Galt and Taldor. The Gray Gardeners would love to reclaim Toothy Morris, but all their attempts to date have failed.1