Mhysa Faer is a city founded by the Valyrian Freehold. Although rich and glorious, it did not have self-rule and was instead governed by men and women sent from Valyria to rule in the name of the Freehold. Mhysa Faer was presumably ruined during or after the Doom of Valyria.
The exact location of Mhysa Faer has not been revealed yet. However, "Mhysa" is a Ghiscari word meaning "mother", suggesting Mhysa Faer may be located in territory of the Old Empire of Ghis, which was conquered by the Valyrians during the Ghiscari Wars.