Is this lore in any of the games?
It's a rough summation of the Pocket guide, third edition.
The Dynasty's rise to power:
Eplear's success in uniting the inherently wild Bosmer must be counted among the greatest military feats in Tamriel's history. The Camoran Dynasty was itself a far different beast than the empires of man that also rose in the First Era. In the forests, after all, diversity was the law, and the Bosmer welcomed escaped slaves from the Ayleids, and then Ayleids fleeing the Alessian rebellion, and finally the new Empire itself, forming a trade treaty in 1E 340 with men who had only recently thrown off the brutal Ayleid yoke.
The fall of the Camoran dynasty to Reman and his Empire, note the date, 2714, right during Reman's reign:
Even with the eventual dissolution of the Alessian Reform of Marukh, battles continued to be waged along the Cyrodiil and Valenwood border lands. When the Empress Herda improved relations with the Colovian West, the attacks only intensified, though it was not until 1E 2714 - after relenting warfare and a devastating plague from the island of Thras - that Valenwood fell to the Cyrodilic Empire. The Camoran Dynasty as an entity still existed, but its power was diminished. The Empire, eager to make sure that Valenwood would not unite against its new occupiers, granted independence to each treethane of the Dynasty, so nevermore would they battle together against a common foe. Falinesti, Silvenar, Haven, Archen, Eldenroot, Woodhearth rose from being local trading posts to full-fledged powers in their own right.
The Camoran Kings are proven incapable of reunifying their homelands, and the Thalmor become the true governing force of Valenwood:
For centuries, the Bosmer were obedient if not particularly loyal subject of the Cyrodilic Empire. At the fall of the Cyrodilic Empire in 2E 430, the Camorans attempted to reinstate their authority over the other kingdoms, but culturally each had drifted too far away to be united. Without any other greater power to rein in their ambitions, they began to war against one another, the Khajiit to the east, and the Colovians to the north. Valenwood ate away at itself, and offered no resistance to the coastal encroachments of the Maormer of Pyandonea. It took another outside force to reunify Valenwood: the home of the ancient Bosmer, Summerset Isle.
The unified elven kingdom of Valenwood and Summerset, the Aldmeri Dominion, was the most stable power in Tamriel until the coming of Tiber Septim. The new government of Valenwood was called the Thalmor, a congress of Bosmeri chieftains and Altmeri diplomats. While not particularly popular, the Thalmor proved better than the chaos of the previous years, and endured until Tiber Septim's armies swept it away.
Tiber Septim allows the Bosmer to keep a figurehead Camoran King, and the rise and fall of the Usurper, Haymon Camoran, supposedly the father of Mankar Camoran according to the book "The Refugees". Notably, the Argonian you consult with in Oblivion during the MQ posits that Mankar Camoran is contemporary with Tiber Septim, which would make him far too old to be Haymon's son.
Wisely, the Emperor allowed Valenwood to keep some of the symbols of her independence, such as the tribal councils and a figurehead Camoran king. For two hundred and fifty years, Valenwood was at peace. The War of the Isle and the War of the Red Diamond, which ravaged other parts of the Empire, left it unscathed. The Empire used the province as it saw fit, and neglected it otherwise. Gradually, the Bosmer began to grow resentful of an authority which seemed increasingly alien - perfect breeding ground for the horror which was to follow.
In the year 249 of the Third Era, a pretender to the ancient throne of the Camorans appeared, and with mundane and Daedric allies, stormed across Valenwood, destroying all who stood against him. The Bosmer were slow to unite against the threat, many too terrified to stand against the Camoran Usurper and some delighted that they were being freed, however violently, from the perceived yoke of the Empire. This minority grew as the Usurper's power did, and once he had consolidated his power in Valenwood, he turned his attentions northward. It took nearly two decades of tyranny before Valenwood found the strength to shrug off Haymon Camoran's rule. When he lost his seat of power, the Usurper's conquests in Colovia and Hammerfell rose in revolt, and his army was destroyed in the Iliac Bay between Hammerfell and High Rock in 3E 267.