Jagar Tharn was the Imperial Battlemage who secretly imprisoned Emperor Uriel Septim VII and used Illusion magic to masquerade as the Emperor for a decade. From 3E 389 to 3E 399, a period since known as the Imperial Simulacrum, Uriel VII was held captive in a realm of Oblivion while his battlemage reigned over the war-torn Empire. He was finally killed, and Uriel VII was freed and restored to the Ruby Throne. The Emperor dubbed the hero who accomplished this the Eternal Champion, and granted him a place by his side. It is not entirely known what Tharn's goals and personal accomplishments were during his reign.
Tharn never appears in Shadowkey but his influence is felt throughout the game as he is one of two mages trying to gain control of the Umbra' Keth (the other being Pergan Asuul). His name first appears when Skelos Undriel reveals that Tharn is hunting him, presumably to remove a shadowmage with opposing views.
During the quest Bribery Papers, it turns out that he is bribing prominent Hammerfell families to side with Skyrim in the ongoing war and that he is also funding Lakvan, a Skyrim raider. Both actions will inevitably lead to more conflict to strengthen the Umbra' Keth.
His plans are thwarted at the end of the Main Quest with the destruction of the Umbra' Keth.