Hlat'tar is a massive cavern of Sygning. In the Horgon Era, it held a deep lake that nearly filled the entire area. The drow of Filion'lyr warred with the area's two aboleth city-states. For centuries, the drow were on the defensive until devising a plan to drain the area. Their plan worked, leaving the aboleth cities half above the water. The drow then poisoned and fouled the shallow lakes around these half-submerged cities. The drow did not let up on Hlat'tar's aboleth, with the last of them being hunted down on 19 Hollow 45.
Hlat'tar is cut by the Vuzlyn river. The river flows past the aboleth ruins Azoloch and Gon'tora and down the middle of the cavern. When the Filion'lyr opened their tunnel that drained the lake, it bored out the river bed to roughly twenty feet under the old lake bottom. This left pools and smaller lakes all about the area which are mostly stagnant except in times of extreme flooding. Hlat'tar is very damp, with plants and animals befitting a subterranean swamp. Since the area is part of the Great Tradeway, roads cut through the area following the river. The major hazards of the area are shambling mounds and the occasional undead aboleth that slithers forth from one of the half-submerged ruins.