The swords are famous among supernatural circles and are always seen in the hands of a Warden. After Luccio's body was stolen, she lost the ability to make swords for new wardens such as Harry Dresden.
Made of silver and supernaturally sharp, they have been seen undoing complex, powerful magic at the will of the wielder; to do so, they must be tailored specifically to each individual Warden. Donald Morgan's sword was also the symbol of the authority given to him by the White Council.
In the series
In Dead Beat, Harry Dresden watches as Luccio wields her sword against zombies. He feels a humming surge of power emanate from it; whenever the blade touches a zombie, there is a flash of light. The sword is later used to destroy a specter.
In White Night, Harry Dresden assumes Luccio never made him a sword because he wasn't trusted, but she explains that she can no longer make the swords after the Corpsetaker forced her into a new body. She doesn't know if she'll ever get the ability back.