Zadnothruin

From LSWiki

Sometimes referred to as "The Underdark." A vast network of caverns with pockets of subterranean civilization. Includes the drow city of Yathryn, the vlekthid city of Syllyac, the huge underground lake Loch Nether, the nyloc outpost of Shadowmyrk, Otan's Cave, Tusker's Den and the Catelius Major.