The Third Era of Tamriel is an important period of time during in the fictional universe of The Elder Scrolls; it is important because it encompasses many of the events of the games in the series, from The Elder Scrolls: Arena to The Elder Scrolls IV: Oblivion.
After the Third Era began, Tiber Septim reigned over the empire for thirty-eight years. On his death bed, Tiber Septim bequeathed his throne to Pelagius. Within a few generations the Septim family line began to develop envy and rivalries until it all spun out of control into a civil war in the year 3E 120. This war was granted the title of the War of the Red Diamond. The war ended when Uriel III died along with his mother, Queen Potema of Solitude. After the war the empire was never again fully unified.
Later in the third era, the Colovian West, Hammerfell and Valenwood suffered from the Camoran Usurper who ravaged their land with undead and Daedric legions for almost twenty years before his fall in the year 3E 267.
During the third era, Uriel Septim VII was imprisoned by Jagar Tharn, who then assumed his identity. Within the next ten years, many wars took place throughout Tamriel. Eventually, Jagar Tharn is defeated by the main player and protagonist in the game The Elder Scrolls: Arena.
The Third Age ended with the death and/or elevation to godhood of Martin Septim, the last of the Emperors. Martin sacrificed himself by shattering the Amulet of Kings, turning himself into the avatar of Akatosh, and banishing Mehrunes Dagon. After defeating Dagon, Martin turned to stone whilst still in the form of the avatar of Akatosh. With Martin's disappearances, there is no remaining heir to the Dragon Throne.