Mnar is the land where the doomed city of Sarnath was located.
The River Ai runs through the territory. Successive waves of inhabitants have come and gone over its long history.
Originally inhabited by the amphibious Thuum'ha who constructed the city of Ib, these creatures were displaced and then rendered extinct in a deliberate genocide by a race of humans who followed the River Ai into the region. These humans built Sarnath on top of the location of Ib.
Behind the Mythos[]
Mnar is often treated as though it is part of the Dreamlands but close reading of the texts does not confirm this. It certainly is ancient, and may have been part of Earth's forgotten prehistory.