Vaerndoun was a network of interconnected caves in the Underdark.
Vaerndoun stretched for over 100 miles (160 kilometers) northeast to southwest with the center point directly under the city of Torsch in southeastern Sespech. Many, but not all, of the caves were filled with water, forming lakes.
Vaerndoun was the site of a two-week trade fair at the beginning of Alturiak every year. It was hosted by mind flayers who referred to themselves as, and claimed to be from, a hidden realm called "the Asglyth". The fair, called the "Eleave" in the drow tongue, was visited by agents from all over the Underdark as long as they provided the admission price of slaves for the illithid to feed on.
The traveling merchant Rebrum of Sheirtalar wrote in the Life of Rebrum (written circa 1210 DR) of an appearance of the Chanting Chain, a holy relic of Talos the Destroyer, at the Eleave. A group of drow traders, sent by their matrons, attempted to barter the Chain to another group of drow from a distant city. However, the Chain likely fell into the hands of one of the Asglyth mind flayers.