Wormwrithings

The Wormwrithings were a series of twisting tunnels located to the north and west of Blingdenstone in the Underdark.

Description
Most of the tunnels of the Wormwrithings were ten-foot diameter tubes which typically intersected each other or natural caverns every one to six miles.

Believed to have been originally created by a colony of purple worms, the tunnels were later the home of many small tribes of kobolds who were beset by many larger predators. Before the drow attack on their city, the svirfneblin of Blingdenstone used to track the purple worms still living in the area, as their excretions often contained precious metals and stones. Due to their racial enmity, the gnomes and kobolds often came into conflict as well.

Locations
Some believed that there was a connection between the Wormwrithings and the surface. They pointed to the fact that the Urlbluk kobold tribe, that lived by the headwaters of the Goblintide River, were ruled by the same matriarchal system as were the various tribes of the Wormwrithings.

There existed a large, gem-rich lava bubble in the heart of the Wormwrithings. This cavern was named by local deep gnomes the "Gargoyle Tube" for the many gargoyle-like creatures encountered in the surrounding tunnels. These gargoyles were, in fact, a group of nabassu that lived in the abandoned dwarfhold of Clan Deepdelve above, also known as the Dungeon of Death.
 * Gargoyle Tube

Much of the area known as the Vast Oblivium was created by the beholder, Karazikar, by using its disintegration eye ray. The creature's lair consisted of ten 30' wide, 100' deep, vertical shafts surrounding a central, 500'+ deep chasm known as "Karazikar's Maw". The Maw and shafts were connected by a series of horizontal tunnels and small caves for the beholder's slaves and followers. Rope bridges across the maw connected many of these tunnels to each other and rope bridges inside the shafts were also used by its inhabitants.
 * The Vast Oblivium

Naturally, purple worms laid their eggs in the Wormwrithings to hatch. Sometimes these areas were the home of other creatures that formed a symbiotic relationship with the worms. The other creatures used the area to hunt for prey that were searching for the valuable eggs, thus protecting the eggs themselves. Giant spiders and other creatures which have no interest in the eggs themselves often served this purpose. Fomorian giants sometimes developed a relationship with the purple worms as well and became caretakers of the clusters of eggs. Purple worm eggs were approximately three-foot diameter, silvery spheres with extremely hard shells. The eggs were often clustered together in groups of six and are suspended from their chamber walls by strands of sticky saliva. They were highly valued by wizards for various arcane rituals.
 * Worm Nursery

Somewhere in the Wormwrithings was a magic portal that connected to a series of tunnels beneath the city of Waterdeep. Later known as the Fireplace Level, these tunnels were originally created by House Goth for their trade endeavors, but were abandoned and sealed up sometime around 1308 DR after a purple worm made its way through the portal and into the them. In 1341 DR the Company of Crazed Venturers and Khelben Arunsun rediscovered the Fireplace Level, the purple worm (frozen in a temporal stasis trap), and the portal. Khelben magically sealed the portal using a gate seal spell "for the security of the city".
 * Fireplace Level portal

Phanlinksal

A settlement located in a long, narrow cavern and built in the form of a castle centered around a stalagmite tower, which served as living quarters for prominent members of the community. Inside the tower, balconies lined the walls, overlooking the central elder brain, which was constantly attended to by nimble-fingered slaves. Other structures in the city included the Axial Plaza, a capture pit, thrall barracks, a gladiatorial arena, and birthing pools.