“Hidden under the sands is the ancient city of Thabes, as well as many buried treasures. The legends of this illusory city lured many men to the desert. However, no ome was ever able to make it back. A Sage once said: Don't go near the death desert of Mamorthod, because there lies a city cursed by darkness.”
—Mystery of the Emblem
Mamorthod (マーモトード, Māmotōo) is a vast desert found towards the north west of Archanea, just north of Khadein. It is known by many as the Desert of Death (死の砂漠, Shi no Sabaku) and it was where Anri first visited on his legendary saga to obtain the Falchion. The desert itself is inhabited by vicious Barbarians and Wyverns. According to rumours, an ancient, yet advanced, civilization known as Thabes lays buried under the desert sands. The treasures of those people remain buried in the sand to this day. To the far north of Mamorthod is the Tower of Thabes. It is said that Naga's final resting place was at Thabes. In Fire Emblem Echoes: Shadows of Valentia, a dungeon called Thabes Labyrinth is there, and is clearly the aforementioned civilization.
Mamorthod would eventually become part of Regna Ferox.
Thabes is an edit of Thebes, which was a capital city in ancient Egypt and could be attributed to Thabes being in a desert.