Undermountain

Undermountain is a multi-tiered dungeon, comprised of several inter-connected chambers and lairs located deep beneath the city of Waterdeep and its namesake mountain. Originally home to the dwarves of Clan Melairkyn, the Underhalls were home to a grand dwarfhold before they were taken over by the "Mad Mage" Halaster Blackcloak and his seven apprentices.
As a general rule, Undermountain is a structure with high ceilings of at least 16 feet and smooth floors made of gray granite. Areas that were created by carving out a single rock were generally decorated in a way that made them look like other more standard areas. That said, exceptions existed. A corridor in Undermountain is usually 10 feet wide and high. The stairs have no railings.
The halls of Undermountain are linked to the surface world in many ways, both physical and magical. Some work only one-way. The dungeon has a number of connections with Waterdeep's sewers and through these, it was indirectly connected to other dungeons. Halaster used his network of portals to bring in monsters from all across Faerûn to restock his dungeon.

History

Aelinthaldaar, the capital city of Illefarn was founded around -8500 DR and first started to dig crypts in the area. The ancient dungeon itself started as a mine and dwelling of dwarves belonging to the Melairkyn clan that formed an alliance with the Elves of Illefarn. In the centuries since the dwarves were driven out it has been the home of Drow, duergar, powerful wizards, criminal groups, and convicted felons.
The most famous of those to control the halls of Undermountain was Halaster Blackcloak, the Mad Mage. He and his apprentices explored the ruins and laid claim to many areas. Over the next thousand years they populated these areas with various monsters, magical experiments, numerous portals and traps. Around 1306 DR the City of Waterdeep started exiling convicted felons to the depths of Undermountain.
Various criminal groups use portions of the ruins as their headquarters as do followers of evil gods. Halastar’s death in 1375 DR and the earthquake that accompanied it not only altered terrain but also upset the balance of power among the various groups. It was barely ten years later when the Spellplague tore through Faerûn and once again drastic change came to Undermountain. Many of the numerous portals in the ruins were destroyed cutting off entire levels. Other portals were warped and seemingly granted access to heretofore unknown levels. Also it is feared that pockets of Spellplague may still exist in Undermountain.

Undermountain & Halaster Blackcloak

Over a thousand years ago, the wizard Halaster Blackcloak whirled to the base of Mount Waterdeep, called there from a distant country by chance or providence. Some say he hailed from the Cradlelands, the nearly forgotten empire that spread humanity across Faerûn, Kara-Tur, and Zakhara, from what is now the Plains of Purple Dust, a wasteland created by conflict between the gods. Whatever his origins, scholars record that Halaster brought with him the Seven, as his apprentices are known, and with their help and his power, Halaster summoned beings from other planes to build himself a wizard’s tower. Halaster ringed his tower with a great wall set with lesser towers for each of his apprentices, and he created fields and farms to be worked by his apprentices and their servants.
For a time, it seemed a good life for the Seven, but as the days wore on, they saw less and less of their dread master. Halaster continued to use fell creatures from other planes for construction beneath his tower, and he kept his dealings with them and the nature of the tunnels from the Seven. At length, Halaster’s tunneling broke into the Underhalls, a complex of tunnels built by dwarves around a mithral mine beneath Mount Waterdeep. The dwarves who built the dungeons, the Melairkyn clan, had long ago been killed or dispersed, replaced by ofttimes warring duergar and drow. Halaster began a crusade against both races, participating in wild hunts through the tunnels with allies called from the Outer Planes. The duergar stubbornly remained until the mithral was largely mined out, but then they left the drow to fight Halaster and his minions alone. Halaster captured or killed the remaining dark elves, entrapping their souls for dark magic or twisting their bodies and enslaving their minds. When the last of the drow were defeated, Halaster Blackcloak tunneled on, ever downward, continuing his strange obsession with delving beneath the mountain.
Finally, when Halaster was absent for more than a year, some of his apprentices ventured into his tower in search of the great wizard. They found traps, monsters, and tantalizing hints about power beneath the earth. Individually they plumbed the depths of the dungeons, encountering ever more deadly traps and more powerful foes. For their efforts, they were rewarded with riches and magic. When the Seven were reduced to five, Halaster appeared to his students and explained that he had built the tunnels to guard his experiments and treasures. He enlisted their aid to make his dungeons even more secure.
What happened after that is unclear, but only one apprentice is known to have left Undermountain, and she fled the area, never to return. Halaster continued to live beneath the mountain, using it as a base from which he traveled to other planes and distant lands, entrapping strange creatures and bringing them back to live as prisoners or guardians in his home. In time, Halaster sought out magic to extend his life, and some say that it was these spells and items that finally drove Halaster beyond all reason. While before he had been obsessed and deranged, his quest for immortality seemed to drive him truly mad. While Halaster quested on other planes and sequestered himself in his tunnels, his tower fell into ruin. When Halaster was yet active in the outside world, his home was considered an accursed place, so settlers in the area largely left the crumbling tower alone. In time, the city now known as Waterdeep came to huddle against the mountain and reach down to the harbor. As the city sprawled outward, it reached and encompassed the ruins of Halaster’s home. Undermountain was known to these early settlers, and they often punished criminals by sending them into its depths.

Durnan and the Yawning Portal

So it was for many years until an adventurer named Durnan delved into the depths beneath the tower and returned, laden with riches, to tell the tale. Durnan demolished the last remnants of Halaster’s above-ground abode and built an inn over the well he had used to descend into the depths. He called his inn the Yawning Portal, and Durnan works there to this day, serving patrons and inviting the brave or the foolish to try their hands at the halls of Undermountain

Magical Features

Undermountain had lots of magical peculiarities that were placed by Halaster. Undermountain's proprietor made it impossible to cast magic that teleported people into or out of Undermountain. Neither would various divination magic work, so none could gather information about what was ahead. However, divination magic that operated through objects, work to some degree. Summoning spells also have limitations: their functionality was limited to snatching monsters that were already present in Undermountain. However, magic items that summoned creatures work, at least as a general rule.
Another effect of the many-layered magic of the Mad Mage is that most of the dungeon radiated magic including the floor, ceilings, and walls. This made spells like detect magic from time to time useless ones because either the radiated magic was so strong that it was impossible distinguish specific things like magic items or because the radiated magic works as some kind of background noise that masked the radiated magic of other things.
Undermountain has some areas where magic simply does not work. The origins of these are not exactly known but suspected to be the result of magical experiments.

Type

Underground / Subterranean