Umbra is an enchanted sword created by an ancient witch named Naenra Waerr for the sole purpose of trapping souls. The soul of a hero called Umbra was used in the swords creation allowing it to retain a mind and will of its own. It chooses its wielder and all those who come into possession of the sword are slowly taken over by it and forced to kill to feed its hunger.
Clavicus Vile had apparently made deals with Umbra and sought possession of the sword. Once in Vile’s realm ‘Umbra’ escaped from the sword and cut away a piece of Clavicus’s power for itself. Clavicus was forced to change the walls of his realm to trap Umbra and prevent it from escaping with his power. Umbra, fearing Clavicus would return him to the sword used his stolen power to conceal himself in a city at the edge of the realm.
In the early fourth era Umbra learned a rift in the realm caused by the first ingenium would become wide enough for him to throw the ‘sword umbra’ through, lessening the threat posed by Vile. With the destruction of the first ingenium Umbra threw out the sword which landed in Mundus and caused the two dunmer who created the ingenium to be sucked into Vile’s realm. Umbra took them captive and agreed to release them if they built another ingenium which would tear away a piece of Vile’s realm that would allow Umbra to escape.