The Rhoynar
A river civilisation of the Rhoyne, drowned by Valyria, whose survivors sailed ten thousand ships to Dorne and changed its law forever.
The fuller tale
Along a river longer than any in Westeros they built cities of stone and painted wood ÔÇö Ny Sar, Ar Noy, Chroyane the festival city ÔÇö and worshipped the river itself as Mother Rhoyne. They were famously peaceful, famously wealthy, and famously good at fighting when finally obliged to, which is a combination that attracts dragonlords.
The wars with Valyria ran through generations and ended the way wars against dragons ended. Prince Garin raised a quarter of a million men, beat a Valyrian army in the field, and was defeated from the air; the Freehold's answer to a river people was to poison the river and leave the ruins to the fog. Chroyane has been the Sorrows ever since, and the greyscale that lives in that fog is the one part of the story the sailors still take seriously.
What survived sailed away. Princess Nymeria took ten thousand ships west, lost most of them, and landed what remained in Dorne, where she married Mors Martell and burned the fleet on the beach so no one could argue about returning. Dornish law still lets an elder daughter inherit ahead of a younger son, which is a Rhoynish idea, and Dorne is still the one part of Westeros that has never quite agreed to be Westeros.
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.