Serpent Deeps

The Serpent Deeps was a small domain of the Underdark beneath the eastern Chultan Peninsula in southwest Faerûn.

Geography
They were an extensive network of caverns lying under the Black Jungles and the Mhair Jungles, reaching from Delselar in Thindol in the west to Mhairhetel in Serpentes in the south and to Lapalgard in Lapaliiya in the east.

Lying in the center was the Lapal Undersea. This was filled by water leaching down from the inland Lapal Sea in the lands above. There was some volcanic activity in the central western side.

Connections to the surface were via deep sinkholes found all around the Mhair Jungles and tunnels in the limestone cliffs beside the River Chun. Flooded lava tubes in the harbor of Tashluta—the caldera of an ancient volcano—descended to the Serpent Deeps.

History
The sarrukh of the empire of Mhairshaulk ( to ) originally created troglodytes to explore the Serpent Deeps. These troglodyte explorers returned with reports of illithids and aboleths and other Underdark denizens. Fearing these creatures could someday threaten them, the sarrukh sent troglodytes back into the Serpent Deeps to serve as sentries watching for major invasions from below. While some efforts were made to seize sarrukh territory, the troglodytes gave the sarrukh enough early warning to foil them, and the Underdark denizens could not cooperate in great enough numbers, and eventually, the sarrukh dismissed them as only minor threats. When the sarrukh lost power and the yuan-ti took over, the troglodytes left Mhairshaulk and descended back into the Serpent Deeps and spread across the wider Underdark.

Inhabitants
By the mid–14 century DR, the Serpent Deeps were inhabited by several tribes of troglodytes who fought with one another, the lizardfolk–troglodyte hybrids known as trens, and even a few sarrukh surviving from the ancient realm of Mhairshaulk.

The sarrukh made their lairs all around the Serpent Deeps, but the majority were clustered close to the Lapal Undersea. They lived in small complexes excavated from the rock, and slumbered in hibernation. Occasionally, their creations, the yuan-ti, desired to consult with them on some matter and sent emissaries, who were generally ignored.

Cinder swarms were common here.