The Second Empire is the second empire of Cyrodiil in the history of Tamriel. It spans two distinct eras: The Reman Dynasty, beginning with Reman Cyrodiil  through the nearly 200-year rule of his heirs up until the death of Reman Cyrodiil III, and the Akaviri Potentate, which ruled during the aftermath of the assassination of the Royal Family and the dawning of the second era. 
Not much is known about the culture of Ald-Cyrod. The land must have been very different from how it is viewed in Oblivion, as until the apotheosis of Tiber Septim into Talos Cyrodiil was believed to be a jungle. We do know, however, that during the reign of the Reman emperors and the following Potentates, Cyrodiil was very clearly divided between it's Colovian and Nibenese residents. Akaviri families were accepted into Cyrodiilic life, and the remnants of intermarriage can be seen in the facial structure of aristocratic Cyrodiils. 
The birth of the Reman Empire is shrouded in myth. It is believed that in the first Interregnum, many lords and warlords fought for the throne, while others searched and prayed to Allessia for the return of the peace brought with the first empire. Hrol, king of what is now known as Old Hroldan, specifically went searching for the spirit of Allessia. He had a vision of her standing with Akatosh, and was seduced by the spirit of Allessia. Hrol is believed to have "DID LOVE UNTO A HILLOCK" in the belief it was Allessia.
Nine months later, the Hill birthed Reman Cyrodiil, who even as an infant spoke with the voice of an Adult. He marched into The Imperial City and was proclaimed a divine emperor. Reman managed to unite the Empire's claims of Skyrim, High Rock, Hammerfell, Nibenay, and Clovia by repelling Akaviri invaders at Pale Pass. Elswyr and Valenwood were taken militarily in the next decade or so. Morrowind never joined the Empire because, as the war between them was concluding, Reman Cyrodiil III and his son were both assassinated.
The Reman Empire fell twice. First it fell at it's peak, with the assassination of all Reman Cyrodiil's heirs. This lead to the slow decline of the empire under the Akaviri potentate, Verdidue-Shaie. In 2E 324, the potentate was assassinated by the Morag Tong and his son, Saverien-Chorak, took power. The new potentate and all his heirs were assassinated in the years to come by the Dark Brotherhood, ending the empire.