Jagar Tharn is the main antagonist and the final (and only) boss of The Elder Scrolls 1: Arena. Tharn was an Imperial Battlemage of uncertain race who betrayed Emperor Uriel Septim VII, imprisoned him in Oblivion, and used illusion to take his place as ruler of Tamriel for ten years, from 3E 389 – 3E 399. He did so using the Staff of Chaos, which he tricked Queen Barenziah of Mournhold into giving him in 3E 376 and split into eight pieces afterward which he had hidden.
He also worked for Mehrunes Dagon and upon seizing the throne replaced the royal guard with Daedra. During his time, Jagar Tharn made a deal with Mehrunes Dagon and summoned him in the Battlespire, triggering the events of the spinoff game of the same name. Dagon was defeated, but the Battlespire was destroyed in the process.
Jagar Tharn finally met his end when the hero of Arena found the eight pieces of the Staff of Chaos and stole the Jewel of Fire, which powered the staff, while fighting the evil mage. This allowed the hero to kill Jagar Tharn and free the Uriel Septim VII, who continued ruling until the fourth game where he was killed by the Mythic Dawn.