Ny Sar is a ruined city in Essos. It was the capital of the Rhoynar before they were forced to flee to Dorne by the Valyrians. The city was destroyed by the Valyrians. It sits at the confluence of the Rhoyne and the Noyne.
In the books
In the A Song of Ice and Fire novels, Ny Sar was the capital of the Rhoynar state before the Valyrians destroyed it with dragons. Queen Nymeria led an exodus of her people from all the lands of the Rhoynar, fleeing into the Summer Sea and thence to Westeros, where they landed in Dorne and allied with House Martell, helping them become the dominant power in the region.
- Ny Sar at A Wiki of Ice and Fire (spoilers from the books)