Mother Rhoyne is a deity associated with the Rhoyne.
The Rhoynar were the greatest of Mother Rhoyne's children, and it is believed that she would whisper the secrets of their enemies to them, allowing the Rhoynish water-wizards to summon walls of water with which to defeat their foes.
Once the Rhoynar were defeated by the Valyrians, Prince Garin the Great was captured, placed in a cage and forced as the dragons slaughtered his people and immolated his home city of Chroyane. Legend states that he called out to Mother Rhoyne to curse the Valyrians, and the city flooded with foul waters and a damp fog which caused their skin to harden and crack, giving birth to greyscale.
In the books
In the A Song of Ice and Fire novels, Mother Rhoyne is the chief goddess of the Rhoynar. She is associated with the Rhoyne, which is revered by the Rhoynar, and it is believed that her waters have nourished the Rhoynar since the dawn of days. The old men of the river are viewed as her consorts.