It was over a thousand years ago that Halaster Blackcloak, the wizard, came to Mount Waterdeep, along with his apprentices The Seven. With their combined power, Halaster summoned beings from other planes and had them build him a tower with a surrounding complex to house them. Halaster continued to use the summoned beings to tunnel beneath the mountain, though the Seven were unaware of his continuing construction. Eventually Halaster broke through into the Underhalls and encountered the encumbent Duergar and Drow residing there; he started a crusade against both the races, pursuing wild hunts through the tunnels with Outer Planes allies. The Duegar remained until the Mithral ran out and then they abandoned the Drow to face the purge alone.
It took little time for Halaster and his allies to defeat the remaining Drow, after which he tunnelled on downwards, continuing with his strange obsession. Whereas Halsater would spend much time out of sight of those on the surface, the Seven decided to investigate his tower after he'd been absent for over a year. Finding traps, monsters and tantalising hints of vast power beneath the earth, they plumbed the depths of the tunnels below, encountering more deadly traps and stronger foes. Their efforts garnered riches and new magics, but the Seven became Five and they returned back to the surface.
After a short time Halaster reappeared to explain to his remaining apprentices, his reasons for the extensive tunnels beneath them. Built to guard his ongoing experiments and accumulated treasures, he wanted their help to make the complex even more secure. It is unclear what happened then, but it is known that only a single apprentice left the Undermountain and she fled never to return. Halaster continued to use the complex as a base, travelling to other planes and distant lands, entrapping strange creatures to expand his menagerie. In time Halaster sought out magics to extend his own life, but whilst he'd been obsessed and deranged in digging out his complex, this quest for immortality seemed to drive him truly mad.
During the time Halaster was travelling in other planes and sequestered underground, his tower complex on the surface fell into ruin. The tower was considered a fell place though and so was largely left alone by settlers in the area. As Waterdeep grew, the city swallowed the complex ruins and knowledgeable of the underground dungeon sprawl, many criminals were punished by being sent into its depths.
Only when an adventurer, named Durnan, delved its depths to return laden with riches, were the ruins finally levelled. He used his new wealth to demolish the last remnants of Halaster's above ground abode and he built an inn on the site, which he named the Yawning Portal.
It is believed that there are nine main levels in the dungeon complex, with a number of additional sub levels; they are split into three groups - the Upper Levels, the Deep Levels and the Gauntlet Below.
The Upper Levels:
The Dungeon Level - long used by the rulers of Waterdeep for dumping undesirables, this level is also known as the Temple Level for the numerous temples built to dark gods over the years.
The Storeroom Level - Halaster enlarged the earlier dwarven caverns and tunnels to house his acquisitions and experiments.
The Sargsuth Level - known for the river running through this massive level. The southern third of the level includes the remnants of the Sargauth Enclave (built by Netherese wizards over 2000 years ago), Skullport [a.k.a. the Port of Shadows], a lawless city of many races that traffic any substance if there is a buyer and Qilue's Promenade of Eilistraee [Promenade of the Dark Maiden], a temple complex NE of Skullport dedicated to Eilistraee. Nearby this level are the abandoned ruins of an ancient Ghaunadaur worshipping drow city.
The more northern two thirds of this level were constructed by Halaster and act as further storage to the level above.
The Deep Levels:
The Farms Level - a strange open level that has a magical branch of the Sargauth river flowing through it. Primarily run by beings from the Realms Below, it produces slimes and algae for food throughout the dungeon complex. Extensive magics move food, water and refuse about the whole complex which make the farms very valuable and thus extremely well guarded.
The Maze Level - rumoured to be home to drow, umber hulks & duergar.
The Seadeeps - these tunnels extend west beneath the ocean, where the dwarves followed rich mithral veins in their mining.
The Gauntlet Below:
The Caverns of Ooze - home to numerous amorphous creatures that serve That Which Lurks.
The Terminus Level - a mix of mines, dwarven chambers and natural caverns; this level is home to many of Halaster's failed or freed experiments.
The Mad Wizards Level - a well guarded level said to house Halasters greatest treasures. Riddled with links to the Underdark, this was the entry point for the drow invasion in past centuries.
The Sub Levels:
The Waterdeep Sewers - not unusual for a large city, the links to the Undermountain can see more frequent visits by strange creatures than the average sewage worker may expect. The lower parts are also known as Downshadow by an increasing number of displaced people and criminals, now living in a large cave at the southern end of the Dungeon Level.
The Citadel of the Bloody Hand - dug into the heart of the mountain, it is now used by the Guard for a secure retreat, should the city come under attack by an overwhelming opponent.
The Shaft - a near 4,000 foot vertical shaft connecting the levels between the Farms level down to the Terminus level. Originally a lava tube, the Melairkyn dwarves reinforced the shaft and used it to move mithral from deep in the mines to the upper levels. Hundreds of side tunnels branch from the Shaft to connect to many main and sub levels around it.
The Dark Levels - a confused group of sub levels between Farms and Seadeeps levels, many are now inaccessible or difficult to reach.
Wyllowood - a flourishing underground forest, nourished by sunlight brought down from the surface, located in a vast cavern carved by a tributary of the Sargauth.
The Lost Level - almost inaccessible save for a beholder guarded portal, this level has a dwarf crypt, a temple of Dumathoin and a drow prison.
The Slitherswamp - a largely unexplored level between Farms and Maze levels.
Muirals Graveyard - originally a drow built sub level, after their defeat, the mage Muiral transformed it into a deadly gauntlet which survives still even after he abandoned it.
The Troglodyte Warrens - a portal accessed level ruled by two warring Troglodyte tribes.
Trobriands Graveyard - originally a home to Tobriands constructs, they escaped during the chaos of the Time of Troubles and now its rumoured to be home to Misker the Pirate Tyrant.
The Obstcle Course - a sub level purely comprised of a long chain of traps, carved from the rock by the will of Tyr.
Arcturiadoom - a level long the domain of Arctris (one of Halasters apprentices), after her death the level became the home to increasing numbers of creatures.
The Crystal Labyrinth - originally a drow constructed chamber, Halaster made it his own for storage of his prized treasures. Formed of transparent crystal, it lies above a coral floored cavern filled with seawater.
Vanrakdoom - claimed by Lord Vanrak Moonstar, for followers of the Night Maiden, he has made it into a sprawling temple of Shar dominated by an army of Shadow Plane craetures that guard it.
The Runestone level - a largely unexplored sub-level lying between the Caverns of Ooze and Terminus level, it is rumoured to have numerous small passages linking it to the Shaft.
Shadowdusk Hold - one of the deepest known sub-levels, which may be connected to The Mad Wizards level to its west.
