Underground City

The Underground City is the city of the Tangai. The city is, as the name suggests, underground. There was once a Tangai City aboveground in the same place, but it is most likely not inhabited when the movie Xorn takes place.

Son of Xorn (unofficial)
The underground city may have been formed after the Wine Wars, but it is also possible that it is ancient.

Xorn
In Xorn, Xorn, Kamil, and the Garnites visit the Underground City to enlist the Tangai as allies, successfully.

Father of Xorn
The Underground City may have been destroyed by the Evil Tangai by the time Father of Xorn takes place.

Description
The Underground City is known to have at least one hallway and a council room.