This is tagged as "history," but could as easily be classed "prehistory", for the origins of the medini date back to some of the earliest human migrations on the planet, before any of them encountered the civilizing influences of the elves or the dwarves. It should be kept firmly in mind that this article describes a migration of tens of thousands of years.
"Medini" is a general term applied to a sub-race of human beings whose migration route from modern humanity's cradle (widely assumed to be Sun Ya) took them across northern seas in primitive boats. These "proto-medini" landed first on the coast of northwestern Korak. A branch stayed there, conquering the proto-peoples who lived there and interbreeding with them to establish the Balcheri people and later, the kingdom of Balkland. The elves of Tinhellerin took note of their arrival and made overtures of friendship once their initial savagery had burned itself out (about five very short generations).
The rest of the proto-medini continued south along the coast, in the next few millennia making the crossing to land on the west coast of the continent of Sylantia. These, too, interbred with the primitive remnants of the proto-peoples who still survived there and began to settle in the rich, fertile river valleys of what eventually became Surmeidan.
Over time, two major groups branched off from the proto-medini at this time. One group, according to legend, were led "under the mountain" on a long quest for new lands by dwarven allies. They emerged from their long pilgrimage via a vast cave complex beneath the great mountain fortress later known as Par Isen that warded a high, arid valley ringed entirely by mountains. These were the first true Medini, forged in the fires of the undermountain and with their cultural identity wrapped around tales of a divine being known as the Guardian Paladin, a dual-aspected deity who appeared either as male, or female, but never both at the same time. These people went on to turn the dry, rocky Plains of Isen into a garden via a system of massive aqueducts, and from there to conquer and settle lands to the north and south in the name of their God. Some further breeding with the less saveage of the remaining proto-peoples is sure to have happened, but otherwise the Medini founded the kingdoms of Vin-Nôrë and Vin-Llamáz, two great early human civilizations.
The other group of exiles left at the same time, via ship. Legend says they were banished for their willful violence and refusal to settle peacefully with their neighbors, but whatever the case, what became of them is unknown. Hints of the fate of "the lost tribe of the medini" only began to surface in CE 576, as the incredible story of a man stranded on Cascadia's shores began to make the rounds among the ruling elite. His claims of a vast island nation lost in the middle of the vast "southern sea" (we still need a name for this) remain unsubstantiated as of year 580.
In any event, the last migration of the medini happened as the Balcheri branch of the medini and the Sylantian branches met, violently, on the southern half of the continent of Korak. Both branches born of that same early root retained their martial natures, though each had become tempered as their civilizations evolved. The Sylantians had the advantage in technology, and so conquered the new land and named it Cascadia. Primarily from Vin-Llamáz, these early conquerors brought with them their particular brand of Paladinism and ideas about the rule of law. The Cascadians spread in all directions, only stopping when they arrived at the Sea of Allagos in the east, and when they reached the end of the fens in the south and the (still unnamed) river to the north.