Septim Empire

The Septim Empire, otherwise known as the Third Empire, was founded by the legendary Tiber Septim. The emperors were those of the Septim Dynasty, with the dynasty's end in the Oblivion Crisis marking the beginning of the Fourth Era.

Foundation
The Akaviri Potentate Versidue Shaie ruled for more then 400 years in prosperity that established many traditions in the Empire. He was assassinated by official Morrowind assassin guild, the Morag Tong. His successor would soon also be assassinated by the Dark Brotherhood, leaving no Akaviri heirs.

After the assassination of the last Potentate, an unremarkable series of Emperors would follow later, until the rise of Imperial General Hjalti Early-Beard, more commonly known as Tiber Septim. He commanded the navy and army of Cyrodiil, and through numerous victories bringing independent states of Tamriel under his control. Tiber's own emperor was assassinated in 2E 854, leaving the Elder Council in charge of the government and him in charge of the Empire. In 2E 896, during the Three Banners War he enacted a truce between Cyrodiil and Morrowind to bring it under his rule as largely independent state, in exchange of the golem Numidium, which Tiber would use to conquer the remaining provinces. He united Tamriel under one Empire claiming throne for himself as the Emperor Tiber Septim I, beginning the Third Era and Third Empire.

The first few emperors after Tiber ruled with peace and prosperity, but withing roughly 50 years, mismanagement and political infighting became the norm. In many cases, there were fights over the throne. The War of the Red Diamond was fought among three surviving heirs. In that time, the Empire started to lose grip in several provinces. It was like that until the reign of Uriel Septim V that the Tamriel was once more reunited. Among other things, Uriel Septim V planned an attack on Akavir, where only death and suffering awaited him.

The next emperors ruled managed to preserve the peace and prosperity.

Imperial Simulacrum and the Oblivion Crisis
The reign of Uriel Septim VII was relatively peaceful one, dotted with occasional strife and near-catastrophe. His Imperial battlemage, Jagar Tharn, imprisoned him in Oblivion using the Staff of Chaos and impersonating him for years, the period of impersonation being named the Imperial Simulacrum. The Emperor was saved by unknown hero, just in time to stop King of Wayrest from conquering Daggerfall. By the time Dagoth Ur threw Morrowind into crisis, there was talk about emperor's failing health.

The Emperor's health was not an issue as it turned out, until an assassin prevented him from fleeing Imperial city after all his heirs to the throne were dead. Their murderers, the cult known as Mythic Dawn, was preparing Tamriel for Mehrunes Dagon's invasion. The Emperor's illegitimate son, Martin, close shut the jaws of Oblivion, shattered Amulet of Kings, and closed barriers between Mundus and Oblivion permanently, sacrificing himself in the process.

The Empire had no leaders after the Crisis, so the Elder Council took full control over it.

Legacy
Ever since the beginning of Fourth Era, the Empire undergo heavy changes. Martin's death, apart from ending the Oblivion Crisis, marked the end of Septim Dynasty. Black Marsh seceeded from the Empire and Morrowind was largely destroyed, with only western part under Dunmer rule, and south by now-independent Argonians. The Elder Council ruled for next fifteen years, until the death of Chancellor Ocato, igniting civil war in Cyrodiil. The war ended when Colovian warlord, Titus Mede I conquered Imperial City and took what was left of the Empire under his rule.

By the time of Great War, the Empire was merely a shadow of its former self. Only by signing the peace treaty, White-Gold Concordat, was the Empire able to survive the onslaught of High Elven Aldmeri Dominion. The Empire lost Hammerfell, the southern part which would've been taken by the Dominion, were not for Redguard uprising. The southern portion of Hammerfell would be at war with the Dominion until Second Treaty of Stros M'kai, declaring the truce between the two forces.

The peace treaty also banned worship of Talos as god, which lit the Stormcloak Rebellion in Skyrim. All that's left of the Empire were the provinces of Morrowind, High Rock, Cyrodiil and war-torn Skyrim.