The Buried Realms was a domain of the Underdark lying underneath the vast desert of Anauroch. The area was called the Phaerlin (pronounced: /ˈfeɪrlɪn/ FAYR-lin) by the phaerimm.
The whole of the Buried Realms was bordered by the Sharnwall. This magical artifice was created in ancient times by the sharn to imprison the phaerimm. It functioned as a barrier, confining the phaerimm to the Buried Realms. They could not travel between their realm and the rest of Faerûn by any skill or magic they were capable of. However, all other creatures could cross it freely.
As Anauroch was once the site of the old Netherese Empire and its successor states, many Netherese ruins could still be found in the Buried Realms. However, the threat of the phaerimm discouraged most adventurers from delving into the Buried Realms, even though they could pass the Sharnwall, so the sites and settlements in the area remained largely unknown to surfacers by the 14th century DR.
On Nightal 20, the Year of the Unstrung Harp, 1371 DR, the magical barrier was breached near the city of Evereska, unleashing the phaerimm to cause chaos in the lands outside. Eventually, the return of Thultanthar and the forces combating the phaerimm kept them at bay.