Carnwennen is a large dagger made out of cold iron and has a hilt made out of bone or ivory. Although it is a dagger it is quite large being almost the size of a short sword.
It is unknown what the full extent of Carnwennens' powers are but it is able to kill a Gray Lord with no trouble.
Carnwennen is a Fae artifact that was orginally was given to King Arthur. He would go on to use it to great effect in killing the Very Black Witch. In the present day it was given to Anna Cornick by the Gray Lords to use to kill Dana Shea (another Gray Lord) via the deceased Sunny Madden. Anna hides the dagger in a coat and then uses the dagger to stab Dana in the heart through her back killing her before Tom Franklin uses Excalibur to behead the Gray Lord to make sure of the Fae's death. Anna would then try to give it and its' partner Excalibur to Jerry the Troll but was instructed to hold onto the weapons for a little while longer.