Naenra Waerr was an ancient witch who enchanted the infamous sword Umbra for the sole purpose to entrap souls. She was eventually executed for her creation, but not before she was able to hide the sword. Some people speculate that Naenra Waerr was in fact Sheogorath in disguise.
Clavicus Vile wanted a weapon that would send him souls. He could not find a smith who could craft such sword, but Vile eventually came across Naenra Waerr. Naenra crafted the sword, but it was unstable. She told the Prince the he had to imbue the sword with part of his own power so that it could be whole and communicate with it on the mortal plane. Vile agreed to do what the witch asked, but she tricked him. The sword would possess its owner over time. The sword ended up being independent from Vile, a sentient being.
- An Elder Scrolls Novel: Lord of Souls (mentioned only)