Umbra Sword (Morrowind)

"The Umbra Sword was enchanted by the ancient witch Naenra Waerr, and its sole purpose was the entrapment of souls. Used in conjunction with a soul gem, the Sword allows the wielder the opportunity to imprison an enemy's soul in the gem. Naenra was executed for her evil creation, but not before she was able to hide the Sword. The Umbra Sword is very choosy when it comes to owners and therefore remains hidden until a worthy one is found."

The main purpose of Umbra is "entrapment of souls." Calvicus Vile said that the sword contains the soul of Umbra, a hero he had been dealing with sometime before and wanted to finish it. Learning from the history, where Clavicus Vile was interested on the N'Gasta's work and also Sorkvild's, which is Necromancy, probably Naenra Waerr had also a Necromancer and had dealt with Clavicus Vile for something. For procuring Umbra, Clavicus Vile offered his Masque to the champion.