"Meanwhile, the first of the Hyborian kingdoms has come onto existence, the rude and barbaric kingdom of Hyperborea, which had its beginnings in a crude fortress of boulders heaped to repel tribal attack. There are few more dramatic events in history than the rise of this fierce kingdom, whose people turned abruptly from nomadic life to rear dwellings of naked stone, surrounded by cyclopean walls." -- Robert E. Howard: "The Hyborian Age"
A northeastern Hyborian kingdom situated east of Asgard, west of Turan, and north of Brythunia and the Border Kingdom. It is presumed that the western boundary of Hyperborea with Asgard was the River of Death Ice, having its source in the Eiglophians. This stream would be shallow, clogged by glacial debris. Since it was also frozen throughout the long winters, it provided an ineffectual barrier against invading Aesir.
The eastern Hyperborean frontier was also quite open, delineated vaguely by low hills, a few lakes, and a small river flowing north from the Graaskal Range or Frozen River. Conan crossed over the mountains into Brythunia, but had a rugged time of it. The principle break in the southern rampart was Skull Gate Pass.
The climate of Hyperborea is presumed to have been milder during its florescence, allowing some simple agriculture as well as herding and hunting. The early chieftains built stone keeps. The lower classes lived in huts or cave-like hovels, scratching out gardens near the protective castles and herding first ponies and reindeer, and later shaggy cattle.
As the lordlings consolidated their positions and Hyperborea became a kingdom, rebellious tribes migrated southward in an invading horde, into the lands of Acheron. It was probably much later that Hyperborea the Elder was overwhelmed by a wave of Mesolithic Aesir -- perhaps after the climate took a turn for the worse and the economy had been weakened by crop failure and loss of domestic animals.
The invading Aesir settled among the ruins, intermarried with the survivors, and created a new nation -- which also called itself Hyperborea. It prospered and eventually attained a high culture -- apparently abetted by a clique of wizards of alien origin.
By Conan's time, Hyperborean slave raiding had added Hyrkanian and Zamoran genes to those of the northern tribes. The people were pale-skinned, extremely tall and gaunt, with white hair and greenish eyes. The Hyperboreans had to fight off not only Aesir, but also Vanir and occasional Cimmerians. The climate was once more taking a turn for the worse and the culture was decaying. Political control was largely vested in a wizard's guild called the White Hand, which tried to ally itself with other occult forces in Stygia, Nemedia, and distant Kambuja.
After the Age of Conan, Hyperborea was conquered by Hyrkanians who galloped around the frozen tundra north of the Vilayet Sea.
References: Hyborian Age I-II, Thing in the Crypt, Lair of the ice Worm, Witch of the Mists.