From A Wiki of Ice and Fire
Mantarys is a city that lies to the north of The Lands of the Long Summer on the very northern tip of the Sea of Sighs. It is connected by Valyrian roads to the ruined cities of Bhorash and Oros. The demon road passes through Mantarys. It has a sinister repute and is allegedly a city of monsters.
A Dance with Dragons
In Meereen Queen Daenerys Targaryen sends a mission to Mantarys hoping to find new friends to the west to balance the enmity of Yunkai to the south. Mantarys's answer comes by way of caravan in a cedar chest - inside she finds the pickled heads of three of her envoys. Instead, Mantarys agrees to an alliance with Yunkai.
References and Notes