The Empire is a contemporary term referring to the area under Imperial rule on the continent of Tamriel. There have been three such Empires in recorded history, and the rule of any given Empire spans centuries. They should not be confused with similar imperial organizations, such as the First Empire of the Nords or the Aldmeri Dominion. While all reigned over great swaths of Tamriel, Tiber Septim's Empire of the Third Era is the only political organization which can boast to have united all of Tamriel under one banner.
 Alessian Empire
The Alessian Empire, also called the First Empire, was formed by the former human slaves of the Ayleid Empire. The Empire was founded when Saint Alessia received the Dragonfires and the Amulet of Kings from Akatosh; she used the Amulet to drive the Daedra out of the mortal realm. Akatosh told Alessia that as long as the Dragonfires remained lit and her heirs wore the Amulet of Kings, the gates of Oblivion would remain shut.
There are few recorded rulers of the First Empire, among them Queen Alessia, its first ruler, religious leader, and empress; Belharza the Man-Bull, Alessia's successor, coronated 1E 266; Emperor Gorieus, coronated in 1E 461; and Herta, who expanded the reaches of the Empire all the way to High Rock. The Alessian Empire expanded in size and power with the rise of the Alessian Order until the Order was dissolved shortly after the War of Righteousness. During this same war, the Empire itself split, with Cyrodiil going into a civil war. The nation was subsequently reunited by Reman I as the Second Empire.
 Second Empire
The Second Empire, also known as the Cyrodilic Empire is the name given to the period at the end of the First Era and the start of the Second when all Tamriel bar Morrowind was ruled by a single native dynasty.
 Reman Dynasty
The Reman Dynasty was founded by Reman Cyrodiil after he defeated the Akaviri invasion of Tamriel at the battle of Pale Pass in 1E 2703. Until that point, Tamriel had been divided into several small kingdoms, but once Reman had brought the armies of Colovia and the Nibenay Valley together to defeat the invasion, he was able to unite all Tamriel's human inhabitants in the face of possible aggression from the Aldmer.
Although the Battle of Pale Pass established Reman as the leading power in Tamriel, many parts of the continent refused to acknowledge his rule. Valenwood was not subdued until 1E 2714. In 1E 2811, Cyrodilic forces defeated the last organized army of Argonians at the Battle of Argonia, and the following year saw Argonia admitted to the Empire as a province under the name "Black Marsh". The rest of the province was conquered piecemeal, with the whole area passing under Imperial control in 1E 2837.
The last war of conquest began in around 1E 2840 when Reman II began an attempt to bring Morrowind, the only province outside Imperial control, into the empire. Progress was slow, and the three living gods of Morrowind's Tribunal, Vivec, Almalexia and Sotha Sil proved to be extremely capable leaders. By 1E 2920, both sides were approaching exhaustion and after the Empire captured the fortress of Ald Marak, a truce was agreed to bring an end to the war.
In the aftermath of the battle, both Reman III and his heir, Prince Juilek, were assassinated on the orders of Potentate Versidue-Shaie, bringing an end to the Reman Dynasty and instigating the period of the Second Empire known as the Akaviri Potentate.
 Akaviri Potentate
The Akaviri Potentate is the name given to the period of the Second Empire after the assassination of Reman III and the end of the Reman Dynasty. Additionally, it is the title given to the rulers of Tamriel both during this period and during the earlier years of the empire.
Under the Remans, the emperor's chief advisor was called the Potentate. In the battle of Pale Pass, Reman I decisively defeated an Akaviri invasion but other events forced him to offer the defeated army amnesty in return for their service in his own army. This led to Akaviri influence in other spheres of life, and over time the position of Potentate was usually filled by one of the snake people. In the year 1E 2920, the Potentate Versidue-Shaie plotted to overthrow the empire. He contrived the assassination first of Crown Prince Juilek and then the emperor himself. On 22 Evening Star 2920, Versidue-Shaie declared that he was taking over the leadership of the empire but retaining his title of Potentate. In the same speech, he declared the end of the First Era and the start of the Second.
In order to suppress armies remaining loyal to the Remans, as well as the armies belonging to individual kingdoms seeking greater autonomy, Versidue-Shaie was forced to engage in a relentless series of campaigns that, by the year 2E 283, finally resulted in victory for his forces.. The cost of the campaigns was immense, resulting in the impoverishment of both the empire and its constituent kingdoms and in an effort to maintain order without the expense of a standing army, in 2E 321 Versidue-Shaie agreed to a proposal from his kinsman Dinieras-Ves that established what was to become known as the Fighters Guild.
In 2E 324, Versidue-Shaie was assassinated by the nascent Morag Tong and succeeded by his son, Savirien-Chorak. In turn, he was assassinated along with all his heirs on one night in Sun's Dawn 2E 430, bringing a final end to the Second Empire and signaling the start of the Interregnum.
 Third Empire
The Third Empire was declared by Tiber Septim in 2E 854 after the chaos of the Interregnum, and was forged through a decades-long conflict to unite Tamriel known as the Tiber Wars. It lasted throughout the whole of the Third Era and two centuries into the Fourth Era, when it began to decline.
 Septim Dynasty
The Third Empire was founded by Tiber Septim in 2E 897 (or 3E 1) when he used Numidium to conquer the whole of Tamriel. The 38 years that followed were prosperous and relatively peaceful. After Tiber's granddaughter Kintyra I was assassinated in 3E 53, the Empire's hold on Tamriel began to slip.
In 3E 121 the empress Kintyra II was imprisoned by Potema Septim, the infamous Wolf Queen of Solitude, and murdered two years later. This triggered the War of the Red Diamond, a civil war between Potema and her son Uriel III on one side, and Potema's brothers Cephorus and Magnus on the other. The armies of the brothers were eventually victorious and the succession continued.
The Empire gained and lost power and land unpredictably during the next century until the rise of Katariah Ra'Athim. The first (and only) Elven ruler of the Empire, Katariah was the first in 100 years to secure a steady, iron grip over most of Tamriel, save Black Marsh. Ultimately, this would be the Empress' undoing, as she died in 3E 200 in a minor skirmish in Black Marsh.
Another civil war broke out after the Elder Council voted to disinherit Andorak, the eldest son of Uriel IV, and offer the Imperial crown to his cousin Cephorus instead, as they considered Cephorus to be more closely related to the true Septim bloodline. The war lasted nine years until Andorak was mollified by the offer of the throne of Shornhelm in High Rock.
In 3E 268, Uriel V began his invasion of Akavir. This one invasion dominated his reign, and ultimately ended it. His life was cut short in 3E 290, and again the Empire began a slow decline. Uriel VI was unable to take the throne until 3E 307, due to his young age at the time of his father's death, and his mother Thonica's attempts to stay in power as Regent. When Uriel VI fell from his horse and died in 3E 320, his sister came to power as Queen Morihatha. Uriel VI had left his sister with rebellious vassals and malcontent citizens to deal with. Nonetheless, she acted decisively and by the end of her reign in 3E 339, Tamriel was nearing a sort of unity. Her son, Pelagius IV -- though struggling at first -- brought the Empire closer to unity than it had been in over 300 years, since the days of Uriel I.
Uriel Septim VII, son of Pelagius IV, came to power after his father's death in 3E 368. In 3E 389, Uriel VII's battlemage, Jagar Tharn, managed to imprison the emperor in a dimension of Tharn's own creation. For ten years, later known as the Imperial Simulacrum, Jagar Tharn ruled in Uriel's place, but showing none of the latter's wisdom. Before Tharn could entirely drive the Empire into the ground, Uriel was rescued and restored to the throne in 3E 399.
During Uriel Septim VII's reign, which lasted 65 years, he united the Empire for the first time in centuries. By the end of the Imperial Simulacrum, Warp in the West, and Vvardenfell Crisis, the Empire was even closer to unity than at the end of Pelagius IV's reign. After his assassination his son, Martin Septim, ended the Oblivion Crisis, the Third Era, and - with no more heirs - the Septim dynasty.
The Empire recovered from Tharn's deceit, but ultimately ended when Martin Septim sacrificed himself in a successful bid to seal the rift that had been opened to Mehrunes Dagon's plane of Oblivion. High Chancellor Ocato became Potentate Ocato when no candidate to replace Martin could be found. Ocato kept the Empire more or less intact in the tumultuous years after the Oblivion Crisis, but was assassinated in or around 4E 10, which ushered in the seven-year Stormcrown Interregnum.
 Mede Dynasty
In 4E 168, Emperor Titus Mede II ascended to the throne. The Empire was at that time a shadow of its former glory. Valenwood and Elsweyr had been ceded to the Aldmeri Dominion, Black Marsh had been lost to Imperial rule since the Oblivion Crisis, Morrowind had yet to fully recover from the eruption of Red Mountain, and Hammerfell was plagued by infighting between the Crown and Forbear factions. Only High Rock, Cyrodiil, and Skyrim remained prosperous and peaceful.
In 4E 171, the Great War began when armies from the Aldmeri Dominion invaded the Imperial provinces of Hammerfell and Cyrodiil after Titus II rejected an ultimatum to make massive concessions to the Thalmor. An army led by Thalmor general Lord Naarifin emerged from hidden camps in northern Elsweyr and assaulted southern Cyrodiil, flanking Imperial defenses along the Valenwood border. Leyawiin fell to the invaders, and Bravil was surrounded and besieged. Simultaneously, an army under Lady Arannelya crossed western Cyrodiil, bypassing Anvil and Kvatch and entering Hammerfell. This army was joined by smaller forces landing on Hammerfell's coast. Imperial troops were forced into retreat across the Alik'r Desert. The Imperial City was beseiged, and fell after the Emperor fled with the remnants of his army to Skyrim. The war ended a year later with the liberation of the Imperial City after the Battle Of The Red Ring, and with the subsequent Imperial acceptance of the White-Gold Concordat. This treaty contained terms almost identical to those rejected before the war, specifically the outlaw of Talos worship, which became one of the major causes of the Stormcloak Rebellion in Skyrim. Hammerfell seceded from the Empire and worked to combat Thalmor forces on its own, gaining its freedom in 4E 180. This left the Empire in a diminished and less stable condition, though it retained control of the provinces of Cyrodiil, High Rock, Morrowind, and Skyrim.