Mag Tureah

Mag Tureah was a country-sized cavern fortress in the depths of the Feydark.

Geography
Mag Tureah contained many structures, particularly iron towers. Due to the vast amount of gems on an onyx backset all around the cavern, it looked just like the clear night sky. The onyx stone was sculpted into pillars, curtains, and buildings. This fortress was the home of the fomorian king, Bres.

Mag Tureah was described as a maze of iron and stone, which was odd for a fomorian stronghold. The fortress held countless portals to and from Toril. They opened, closed and collapsed at any time, meaning a bold adventurer could be trapped on a world which wasn't their own. More than one of these adventurers was unlucky enough to go through a portal to Mag Tureah, only to become a slave to the fomorians.