Great Bhaerynden lies in the south-central region of Faerun, beneath the vast plains of Shaar. This Underdark region was the first homeland of the dwarves, but was conquered by the Drow when they first ventured in the Underdark in search of a realm of their own. The Drow called their new home Telantiwar, but this kingdom didn't last.
For still unknown reasons, the domed ceilings of the enormous vaults in which Telantiwar was established collapsed down on the kingdom. Many of the Drow were crushed beneath rocks and rubble from the rock layer above. This great collapse lay ruin to many of the Drow cities of Telantiwar, and created the Great Rift.
In modern days the great cave that housed Bhaerynden no longer exists or is impossible to access due to the fallen rock, it was part of a larger network of caves and tunnels. Gold dwarves now hold the Great Rift and a lot of the adjoining caves. Two Drow races hold cities in the vicinity. Particular powerful Drow cities in Great Bhaerynden are Guallidurth, LLurth Dreir and Tlindhet.
See also: The Great Rift and its connection to Great Bhaerynden, which is also known as The Deep Wild.