Thras and Pyandonea have been implacable enemies of Summerset and parts of western Tamriel for thousands of years, but the deadliest adversary - and most influential alien culture since the coming of the Aldmer and Atmorans - has been Akavir. The mysterious land to the east of Tamriel has been our opponent numerous times, and still we know little about them.
The first appearance in history was as pirates, which we may now assume were also scouts for the eventual invasions. Ships manned by bizarre beastfolk bewildered and horrified the earliest inhabitants of Tamriel. Contemporary scholars find references to pirates with rat-like features, and still others who appeared canine, suggesting Akaviri cultures yet undiscovered and perhaps extinct. Then, of course, there are the dragons, Akavir's most deadly and beautiful former native child. The very name "Akavir," in fact, means "Dragon Land."
In the 2703rd year of the First Era, Tamriel first faced an organized armada of Tsaesci, the so-called "Akaviri Snakemen", and met the challenge with a resounding victory in the Pale Pass of Skyrim. The Emperor Reman was so impressed by the exotic weaponry and battle prowess of his defeated foe that he and his heirs allowed them to rise from prisoners to advisors of the second Empire of Tamriel. Eventually, they became, on the death of Emperor Reman III in the year 2920, the Potentates of Tamriel, and the defeated Akaviri ruled the land for over 400 years.
It is from this era that we know most of what we do know of the land of Akavir. In addition to the Tsaesci, Akavir is said to be home to several other bestial and peculiar creatures: the monkeys of Tang Mo, the snow demons of Kamal, and the tiger dragons of Ka Po' Tun. It should be noted that these various races of Akaviri have never been sighted by modern scholars. While tales that survive for the Akaviri Potentate describe these races in detail, it is unknown how literally they should be taken, given the possible mistranslation of the complex Tsaesci language.
Tamriel met Akavir's aggression many centuries later when Emperor Uriel V, a military genus by any measure, was so ambitious as to attempt an invasion of Akavir based on the Tsaesci intelligence of the Second Era. This proved to be a catastrophe, cumulating with the loss of the Emperor and his entire army in Akavir at the Battle of Ionith in 3E 290.
While our battles with Akavir are likely far from over, their influence on Tamrielic culture has not been uniformly negative. The exotic weaponry and armor they used in the First Era has been adopted by warriors throughout the continent, and their dragon has been adopted as one of the most potent symbols of the Septim Empire.