From Morte d' Arthur:
Then Sir Arthur looked on the sword, and liked it passing well. Whether liketh you better, said Merlin, the sword or the scabbard? Me liketh better the sword, said Arthur. Ye are more unwise, said Merlin, for the scabbard is worth ten of the swords, for whiles ye have the scabbard upon you, ye shall never lose no blood, be ye never so sore wounded; therefore keep well the scabbard always with you.
Given that in Dresdenverse:
1) Excalibur is a Sword of the Cross, and
2) Nicodemus has a Noose with the powers of the Scabbard above, and
3) the Swords can change form -
maybe the Noose used to be the Scabbard? Certainly Nicodemus would enjoy persuading the Knights that one of their holy relics was an evil artifact. It is Arthurian canon that the scabbard is stolen by a witch.