The Kings of the Rivers
House Mudd, then Justman, then Teague, then Blackwood, then Bracken ÔÇö the riverlands have been conquered by everyone who ever wanted them, because they have no borders worth the name.
The fuller tale
The trouble with the Trident is that it is the best land in Westeros and has a mountain range on none of its sides. The First Men kings of House Mudd ruled from Oldstones for a thousand years and were thrown down by the Andals; the Andal Justmans lasted three centuries; the Teagues were burned out by their own lords with ironborn help. The riverlands have never once ended a war on the winning side while it was still being fought on their fields.
Harwyn Hardhand of the Iron Islands took the whole of it three generations before the Conquest, and his grandson Harren the Black spent forty years and a great many river-born lives building Harrenhal to sit on it from. The riverlords cheerfully changed sides the moment Aegon offered them the chance, which is the single most riverlands thing in this history.
House Tully were minor lords through all of it, elevated at last by Aegon for being the first to declare. Three hundred years later the rivers burned again, for the same reason as always: because they lay between two people who wanted to fight each other.
The ground they held
 See the world in 61 BC on the timeline What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.