Thraa is a city in the land of Mnar on the River Ai in the Dreamlands. It was built by the same wandering tribes that built the Mnaran cities of Ilarnek, Kadatheron, and Sarnath. Sarnath eventually conquered the neighboring cities and lands (including Thraa) and forged an empire. (HPL: "The Doom That Came to Sarnath")
Pearls from the coastal region of Mtal dissolved in vinegar produced in Thraa were considered a delicacy in Sarnath. (HPL: "The Doom That Came to Sarnath")