The Andals
Iron-armed invaders out of Essos who brought the Seven, a new alphabet, and the end of the First Men's south.
The fuller tale
They came from the hills of Andalos, on the far shore of the narrow sea between what would become Pentos and Norvos, and by their own account they came because the Seven told them to. The maesters, unromantic to a man, note that they came in the direction away from Valyria and its dragons, and draw the obvious conclusion.
They landed in the Vale first and worked outward, kingdom by kingdom, with iron against bronze and a faith that had no use for a tree. Where they could not conquer they married, which in the long run worked better: within a few centuries almost every great house south of the Neck could point to an Andal in its line, and the ones that could not ÔÇö the Blackwoods, chiefly ÔÇö made a point of saying so.
The Neck stopped them, and the North kept the old gods, which is why the map of the Faith and the map of the weirwoods still divide Westeros along the same swamp today. Their other gift was writing: the runes of the First Men said little more than a name, and the histories of Westeros properly begin when the Andals arrived with an alphabet and the habit of using it.
The ground they held
What became of it ÔÇö if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.