He led a coup against his cousin Kintyra II, whom he accused of being a bastard child. The three armies of High Rock, Morrowind, and Skyrim invaded the Imperial-held areas of Tamriel. After Empress Kintyra's capture and supposed murder in 3E 120 (in reality she was kept captive until she was murdered in 3E 123), Uriel III faced a lengthy period of waiting as his armies attempted to conquer the Empire. The conflict was soon to become known as the War of the Red Diamond, on account of the Septim's famous badge. On one side was Pelagius II's surviving children, the Empire, and most kings of Tamriel, and on the other Uriel III, his mother Potema, Skyrim and northern Morrowind.
Though the Imperial City fell within two weeks, many other parts of the Empire had proven difficult to conquer — as such it was not until 3E 121 that Uriel formally crowned himself as Emperor. He took the name Uriel Septim III, even though his true name was Uriel Mantiarco from his father. Fighting continued in earnest for years, with the scales ultimately tipping against Uriel III.
Uriel III was captured during the Battle of Ichidag in Hammerfell in 3E 127  and was subsequently charged for his crimes against the Empire. En route to his trial, however, an enraged mob overtook his carriage and burned him alive within it. Uriel III's death marked the end of the conflict and the beginning of his successor Cephorus Septim's reign.
- There is a dispute about the date that Uriel Septim III began his rule, with different historians stating that it began on different years.
- A statue of Uriel III resides in the Market District of the Imperial City. The statue is viewable in The Elder Scrolls IV: Oblivion.
|Empire of Tamriel|
|Kintyra II||3E 121 - 3E 127||Cephorus Septim|