Originally created as the Umbra Sword, Umbra is either a part of Vile that gained its own sentience within the sword, by accident or design, or a being that Vile somehow trapped within the sword. It ultimately broke free of Vile's will, creating a body for itself but remaining trapped within Vile's Realm. In order to escape, it forged a pact with the Dunmer mage Vuhon, providing the power necessary to create the floating city Umbriel.
As the Umbra SwordEdit
The Umbra Sword was created by Naenra Waerr to trap souls, at Clavicus Vile's request. As part of the creation process, Vile embued it with part of his essence. This essence would ultimately overtake the wielders and become the driving force in their lives. Several wielders noted that their lives became nothing other than a quest to use the sword in constant battle. Some even named themselves after the sword, possibly foreshadowing its eventual independence.
Over time, either through the process of trapping so many souls or by the design of Vile or Waerr, the sword gained its own sentience. Umbra then escaped Vile, and became part of Vuhon, granting itself independence from the sword.
As an independent spiritEdit
Shortly Vile received the Umbra Sword from the Hero of Kvatch, Umbra escaped the sword and stole some of of Vile's power, greatly weakening the Daedric Prince. The Umbra spirit fled with the sword to one of the cities in Vile's realm to avoid being trapped again, before returning the sword to Tamriel, making it harder for Vile to return it to the sword.
Umbra made a deal with Vuhon to escape to Tamriel, which caused them to merge into one being, creating a new Ingenium and the floating city of Umbriel out of a portion of Clavicus Vile's realm of Oblivion. Umbra and Vuhon tried to get the city to the White-Gold Tower to complete its separation from Vile. However, he decided to take the city north into Morrowind to find the Umbra sword, as it could be used to destroy both Vuhon/Umbra and the Ingenium.
Ultimately, Sul and Attrebus were able to find the Umbra Sword, and use it to destroy the Ingenium, causing the re-absorption of Umbra into the sword, before the sword itself disappeared once more.