Imperial Simulacrum is the official term for the events that took place when Jagar Tharn took the Imperial crown from Uriel Septim.
Jagar Tharn was a mage-priest at the Temple of Sethiete in Camlorn, High Rock before his ascension to the position of Imperial Battlemage.
In 3E 376, he tricked Queen Barenziah of Mournhold to show him the way to Old Mournhold, where the Staff of Chaos was hidden. He then stole the staff and disappeared.
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. 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. Using his powerful, and skillful knack for Illusion, he took on the visage of Uriel VII, and ruled the Empire, replacing the Emperor's guard and servants with Daedra.
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 is 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 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 Azra Nightweilder, thwarted his plan.
His reign is 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 was rescued, and the hero was declared as the Eternal Champion.