The Aldmer were among the first to migrate to Cyrodiil, from their original home of Summerset. The Aldmer who settled in Cyrodiil became the Ayleids and formed an independent state. These elves were known to be brutal dictators, and maintained an active human slave trade. The Ayleid, unlike their cousins the Altmer and Bosmer, were known to deal favorably with the Daedric Princes, using their help and their minions to repress the humans.
With the rise of a stable human empire to the north, and internal conflicts among the Ayleid lords, the human slaves of Cyrodiil saw an opportunity to take control of their homeland from their masters. They were led by a charismatic human woman named Alessia, who was dubbed The Slave Queen. In 1E 243, Alessia led an open revolt against the Ayleid, aided by Nordic allies and, according to some accounts, by renegade Ayleids. Alessia ultimately defeated the Ayleid armies, and took control of the White Gold Tower in what was renamed Imperial City.
Alessia would eventually retool the Cyrodiilic pantheon, incorporating many of the Nordic gods into the existing Ayleid pantheon, and removing the Daedra. The resulting state religion, the Eight Divines, has lasted for thousands of years, and has become the de facto official religion of Tamriel. In addition, decades later as Alessia was nearing death, Akatosh himself appeared to Alessia and gave her the Amulet of Kings to protect Tamriel from invasions out of Oblivion.
The early Empire was much smaller than modern day Cyrodiil, consisting primarily of the Nibenay Basin (called The Heartlands), the Great Forest to the immediate west, and the Jerall Mountains near the Skyrim border. The Empire expanded quickly, however, taking over the Valus Mountains near Morrowind, and the Blackwood region near Black Marsh. The remainder of modern Cyrodiil, including the Gold Coast, West Weald, and Colovia Highlands, were part of the independent empire of Colovia, which Cyrodiil slowly began to absorb over the following centuries.
The early days of the Empire were spent mostly fending off invasions. An Altmer dynasty, the Direnni, rose in power as the last major mer dynasty in human lands. At the time, the Nords, were having internal problems, with their last King having died with no clear heir. During the War of Sucession that followed, the Nords lost control of all of their territory outside of Skyrim, including parts of the Jerall Mountains in Cyrodiil, the portions of Morrowind they had managed to take from the Dwemer, and the High Rock region where the Direnni took root. The Direnni launched a number of attacks on both Skyrim and Cyrodiil, and made headway into the Colovian empire, until finally around 1E 500, the Direnni are defeated.
The last major expansion for the Cyrodiilic Empire proper came under the rule of King Reman I, near the end of the First Era. Approximately 1E 2700, the Akaviri invaded Tamriel and began pushing toward Cyrodiil, intending to conquer the continent. Reman I rallied the peoples of Cyrodiil, and permanently united with Colovia, to drive them out. This began the so-called "Second Empire" of Cyrodiil, and the push to conquer other sovereign provinces. The Empire took control over the region now called the Black Marsh and created it as an Imperial Province, and turned its sights on Morrowind.
By the year 2920, the King Reman III was having moderate success invading the lands of the Dunmer. The King was being advised by a remnant of the Tsaesci of Akavir, ostensibly wishing to serve him after seeing their fathers defeated in battle. In truth, the Akaviri were still plotting to take control of Tamriel, and were either directly or indirectly responsible for the assassination of Reman and all remaining heirs to the throne. Upon the death of the last remaining heir, the Akaviri Potentate, Versidue-Shaie, claimed control of the throne for himself, claiming to be at the behest of the late Emperor himself. This event would mark the end of the First Era, all of which is described in the series of 2920 books.