Later, he managed to become the Imperial Battlemage of Uriel Septim VII. In 3E 389, he captured Uriel VII and banished him into Oblivion with the Staff of Chaos. He then took the Emperor's visage and usurped the throne of Tamriel, replacing the Emperor's guard and servants with Daedra and other foul creatures. The only issue with this plan was that should the Emperor die in the plane of Oblivion, Tharn's treachery would be revealed. Normally when the Emperor of Tamriel dies, the Elder Council is magically informed by the Amulet of Kings and meets to select a successor. By sequestering Uriel Septim VII in another dimension, where time ran slower, Tharn prevented this from happening. He then broke the staff into eight pieces and hid them in the other provinces of Tamriel.
Several years later, he made a bargain with Mehrunes Dagon to invade Battlespire, the Imperial training facility and bastion of the Imperial Battlemages. This move was essential to secure his position and to eliminate a possible threat to his reign. Mehrunes Dagon was destroyed and cast back into Oblivion by a mysterious hero, but, the Battlespire was destroyed. The Imperial Battlemages, a potential threat to Tharn, were no longer capable of thwarting Tharn's reign. 
In 3E 397, Tharn and Pergan Asuul tried to manipulate the War of the Bend'r-Mahk to create the Shadow of War, the powerful creature known as Umbra' Keth. However, another mysterious hero with the help of Skelos Undriel and Azra Nightwielder, thwarted his plan. 
His reign ended in 3E 399. The Eternal Champion, with the help of Ria Silmane and Lady Barenziah, managed to gather the hidden pieces of the Staff of Chaos. During the final battle, in the depths of the Imperial Palace Dungeon, the champion managed to grab the Jewel of Fire, a jewel that held Tharn's power and the whole energy of the Staff of Chaos. Jagar Tharn was slain, Uriel Septim VII rescued, and the hero saw himself declared as the Eternal Champion.
- According to certain backstories for your character in Daggerfall, there were circles of battlemages who were aware who Uriel Septim "really was" But none were brave enough to stand up against Tharn.
- The Elder Scrolls: Arena
- An Elder Scrolls Legend: Battlespire (Mentioned only)
- The Elder Scrolls Travels: Shadowkey (Mentioned only)
- The Elder Scrolls IV: Oblivion (Mentioned only)
- The Elder Scrolls V: Skyrim (Mentioned only)