Undermountain

Overview
This ancient dungeon started as a mine and dwelling of dwarves belonging to the Melairkyn clan. 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 1306DR 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 1375DR 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 oft-times 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. 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

Timeline

 * -1288 DR: King Melair I discovers mithral ore underneath Mount Waterdeep and creates a home for his Melairkyn, beginning to carve out caverns and tunnels
 * -750 DR: Netherese wizards occupy the third level of Undermountain and seal it off against the Melairkyn. Some scholars call this the Sargauth Enclave
 * -677 DR: The drow begin to occupy more and more of the area held by the Melairkyn
 * 34 DR: In a large shift of territory following a massive drow invasion, the Melairkyn lose a lot of their territory
 * 168 DR: Halaster Blackcloak claims the Underhalls of the ousted Melairkyn for himself.
 * 171 DR: Halaster begins Halaster's Hunts which wipe out large numbers of the drow and duergar from Undermountain and continue until 308 DR. Many drow and duergar are forced out during this time.
 * 268 DR: The mithral is now completely depleted
 * 284 DR: The duergar leave Undermountain, having no reason to stay without the mithral resource
 * 309 DR: Halaster becomes ruler of the Underhalls
 * 493 DR: The drow leave Undermountain entirely
 * 1375 DR: Halaster Blackcloak dies.
 * 1385 DR: The Spellplague hits Undermountain, drastically altering the landscape of the dungeon.

Entrances & Exits
The halls of Undermountain are linked to the surface world in countless ways, both physical and magical. Halaster uses his network of portals to bring in monsters from all across Faerun to restock his dungeon. He also uses portals to draw adventurers into the deeper regions, rarely explored, and to give the occasional lucky adventurer an unexpected escape. Some of the notable physical links are described below.


 * Blue Mermaid Passage: This connects a dry well in the cellar of the Blue Mermaid tavern to the Port of Shadow.
 * Castle Corkscrew: This passage corkscrews down from the cellars of Castle Waterdeep to Skullport, bypassing the first two levels.
 * Falling Stair: This connects the dungeons of Castle Waterdeep and the Citadel of the Bloody Hand with Undermountain's first level.
 * Long Dark Stair: This links the oubliette of the Blushing Numph festhall with Undermountain's first level.
 * South Seacaves: This links the Sea of Swords on the western flank of Mount Waterdeep with the River Sargauth by means of a set of hoists.
 *  The Yawning Portal Inn: Built atop the ruins of Halaster's Hold, The Yawning Portal Inn contains the primary entrance into Undermountain, a large well that drops down into Undermountain's Dungeon Level. Secondary entrances are found in the inn as well, including a link to the inn's less well known "wee" well and a passage leading out of the Yawning Portal's cellar.

Levels of Undermountain
Major Levels
 * The Dungeon Level: The Dungeon Level has long been used by the Lords of Waterdeep as a dumping ground for undesirables, hence its most common name. It is also known as the Temple Level, for the many temples of dark gods that have been constructed here over the years, and the Tomb Level, for the many lords and kings that have been interred herein as well. As explorers of Undermountain often visit this level, its features are common tavern-talk of Waterdeep. Notable features of the Dungeon Level include the Bone Throne, the Cavern of Eyes, the Falling Stair, the Grim Statue, the Hall of Sleeping Kings, the Hall of Three Lords, the House of Pain, the Lanceboard Room, the Sundered Throne, the Temple of Gulkulath, and the Temple of the All-Seeing One.
 * The Storeroom Level: On this level, Halaster and his apprentices excavated the rough stone caverns and mining tunnels left by the dwarves to house their acquisitions and magical experiments. Notable features of the Storeroom Level include the Doomgate, the Hemwatch and Lord Hund Hillgauntlet's Tomb.
 * The Sargauth Level: Known for the River Sargauth, which winds through this massive level, Undermountain's third level consists of two major sections. The southern third of the level consists of the remnants of the Sargauth Enclave, now split by a massive cave-in leading to the Port of Shadow and the Promenade of the Dark Maiden. The northern two-thirds of this level were constructed by Halaster and his apprentices, and served much the same role as Undermountain's second level. The most notable features of this level are the Lair of the Eye and the rubble-filled Pit of Ghaunadaur, a mile-deep shaft that leads from the Prime Material Plane into the Cauldron of Slime through an intermittently existent planar breach.
 * The Farms Level: The fourth level of Undermountain contains farms no surface dweller would recognize. Halaster's mightiest magic brought the River of the Depths, also known as the River Sargauth, to this level, where it links huge caverns in broad, sweeping curves, and is navigable by barges. In these caverns, slaves and servants of the drow, aboleth, and other races of the Realms Below produce slimes and algae for food. Halaster controls his own farms here, too, and their produce is teleported to "feed-bin" rooms throughout the dungeon (rooms known to resident monsters as "watering-holes"). Undermountain's privies are also rinsed by magically pumped or redirected water into miniature portals that bring their contents to the farms. The dung-heaps produced by this process are inhabited by advanced otyughs and gulguthydras, among other foul scavengers. All of these farms are quite valuable, as there are few large sources of good food underground; thus, all are heavily guarded. The drow and Halaster have both used fearsome monsters as guardians, including death tyrants, advanced nagas with many sorcerer levels, dragon turtle sorcerers, and even wilder creatures. A large, eerily dark lake at one end of the level supports a kuo-toa "fish factory" of the blind white fish of the depths, an important industry, heavily guarded to prevent food-raids from other creatures of the Underdark with the aid of an advanced kuo-toa leviathan. And island in the middle of the lake is home to an advanced otyugh, piled deep in dung, offal, carrion, and other refuse that finds its way down long, twisting garbage-shafts from Skullport.
 * The Maze Level: This level contains five major features of note - the drow outpost of Kyorlamshin, the Lair of the Umber Hulks, the Maze of Madness, an old dwarf hold built by Clan Melairkyn and expanded by the duergar, and the winding channels of the River Graymurk. Kyorlamshin is linked by a series of portals to the city of Karsoluthiyl, which lies deep beneath the ocean floor west of Baldur's Gate. The drow of House Tanor'Thal continues to dominate both settlements and the slave trade that links them. The Maze of Madness is the creation of Halaster, stocked with ravenous minotaurs and magic traps to confuse and trip up the unwary.
 * The Seadeeps: This level once housed two major mining operations of Clan Melairkyn. Nigh-endless mining tunnels stretch westward, following two now-vanished veins of mithral beneath the ocean floor. Only numerous dwarf-built locks and pumps prevent the ocean waters from seeping down and wholly flooding the level. The Seadeeps have numerous connections to the Swordsea Depths, as the Underdark beneath the Sea of Swords is known. At the present time, the Swordsea Depths are host to a long-running three-sided battle between an alliance of svirfneblin and tomb tappers, drow troops from Karsoluthiyl, and certain aboleth and their evil servant creatures. The Savants of the Dark Tide are refugees from this conflict.
 * The Caverns of Ooze: Largely avoided by the Stout Folk of Clan Melairkyn in their day, the Caverns of Ooze have long been home to all manner of slimy creatures serving That Which Lurks. Countlee slimes, oozes, jellies, and other amorphous creatures frequent these natural caverns. For centuries, a colony of ghaunadans has attempted to transform this level into a massive temple of Ghaunadaur, but Halaster has continued to frustrate such efforts. Many suspect that the inhabitants of this region emanate from the bottom side of the planar breach that lies at the bottom of the Pit of Ghaunadaur and make their way down through a warren of narrow tubes, but such a connection has never been proven
 * The Terminus Level: The Terminus Level is a mix of dwarf-built chambers, now-depleted mithral mines, and vast natural caverns. Many of the one-way portals used by Halaster to restock Undermountain with dangerous monsters open into the sprawling caverns of Undermountain's eighth layer. Additionally, the Mad Mage's failed or free experiments also wander this level. The Terminus Level connects with the heart of Undermountain through the Shaft.
 * The Mad Wizard's Level: Built by Clan Melairkyn as a trading outpost, Undermountain's ninth level is linked to the Underdark by a large number of sinkholes, each ringed by fortifications allowing a small number of defenders to hold off almost any invader. This level was one of the first to fall to the drow, who then quickly made their way up The Shaft into the heart of Undermountain. Said to hold the fabled treaure of Halaster the Mad, this level is whispered of in Waterdhavian taverns late at night by those who dare to dream of plumbing the depths of Undermountain. Of course, Halaster himself has spread many of these rumors, in hopes of inducing more adventurers to die foolish deaths in his trap-filled Underhalls. In reality, Undermountain's ultimate level is one big gauntlet of nasty traps and horrifying monsters. The waiting treasure is also the subject of much speculation. Some whisper that vaults full of hundreds of mighty magic items and countless gems lie hidden in the depths of Halaster's Lair, while others claim that Halaster's greatest and cruelest joke is that the legendary hoard is no more than a pittance.

Sub-Levels
 * The Waterdeep Sewers: Typical of many large cities, Waterdeep has its share of sewer passages beneath its streets. What is not typical of these sewer tunnels however, are the links to the famed Halls of Undermountain which ensures a constant supply of creatures to inhabit them and an even more constant supply of adventurers to combat them. Many adventurers also use the sewers as an access point, bypassing the more notable entrances to the dungeon complex below.
 * The Citadel of the Bloody Hand: Dug into the heart of Mount Waterdeep, the Citadel of the Bloody Hand has until recently been occupied by the Guard. Since Halaster's Higharvestide, the Citadel of the Bloody Hand has become the preserve for the living spells that escaped the Mad Mage.
 * Wyllowood: An apprentice of Halaster created the Wyllowwood over a century ago for his beloved, an elf druid named Wyllow. Set in a cavernous region carved by a tributary of the River Sargauth, this level is dominated by the Wyllowood, a thriving forest nourished by sunlight filtered through a massive portal in the "sky" above. Other notable features include a temple of the Destroyer and a long-abandoned lakeside village built by the Stout Folk of Clan Melairkyn.
 * Maddgoth's Level: Maddgoth's level is a small sublevel famed for the presence of Maddgoth's castle. Built by Halaster, this miniature, floating castle is enveloped in a reduction field that reduces all creatures within it to one-twelfth their normal size
 * The Lost Level: Entombed in solid rock, the Lost Level is almost inaccessible, and its primary egress is said to be a portal guarded by the "Ring of Death," a waiting ring of death kiss beholderkin, death tyrant beholderkin, and the beholder lord Nagus. The Lost Level is divided into three sections - a temple of Dumathoin and crypt for Clan Melairkyn's rulers, a drow prison now dominated by Huntmistress Dhusarra, and a massive cavern carved by a branch of the River Sargauth and now split by an earthquake-caused tremor.
 * The Slitherswamp: Located deep beneath the city of Waterdeep, the Slitherswamp is a largely unexplored sublevel between the Farms Level and the Maze Level.
 * Muiral's Graveyard: Muiral's Graveyard is a sublevel built by the drow soon after their initial invasion that included living quarters and a warrior academy. Muiral transformed these chambers into a trap-filled gauntlet haunted by giant spiders, undead, and driders. Although the monstrous mage has abandoned this area, his deadly creations remain.
 * The Troglodyte Warrens: This region of caves is accessible only by a two-way random portal. It is home to two warring tribes of troglodytes - the Urggroth-Kyan clan and the Szygmog-Thurgin clan.
 * Trobriand's Graveyard: The Stout Folk originally mined this sublevel for mithral, leaving it, by chance or for mysterious reasons, in the crude image of a scorpion. Before Halaster's Higharvestide, the Realm of the Metal Masters was home to Trobriand's rejected constructs, built before he perfected his scaldars. By Halaster's Higharvestide, Trobriand's cast-offs had organized themselves into a self-governing community for mutual defense, and they took advantage of the chaos to escape through a portal to their master's secret sanctum and claim it for themselves. In the wake of their departure, this level was largely abandoned, but it didn't remain that way for long. After being driven from Undermountain by the Eye, Misker the Pirate Tyrant fled to this level and made it his own. Now a portal links the beholder's lair to the Ilzimmer Villa, and House Ilzimmer has begun storing its ill-gotten gains herein.
 * The Obstacle Course: The Obstacle Course is a sublevel composed of a long chain of trap-filled rooms. Carved from the earth by the will of Tyr, this sublevel is intended as a potentially lethal test for conceited, cocky braggarts who do not respect the dangers of adventuring.
 * Arcturiadoom: This level was long the private domain of Arcturis, one of Halaster's most ambitious apprentices. After her death during Halaster's Higharvestide, contingency magics whisked her corpse to her sanctuary, leaving the illusion of a body in their wake. Thanks to careful preparations, Arcturia was reborn as a vile form of undead. She now works to restock the monsters guarding this level, which is now known to a handful of daring explorers as Arcturiadoom.
 * The Crystal Labyrinth: The drow constructed this chamber as a training ground for their young in the Year of the Blue Ice. Halaster discovered it in the Year of Secret Slaughters and made it his own, with transparent crystal walls, floors and ceilings, and a coral-floored cavern filled with seawater directly below. At the heart of the labyrinth lies Halaster's Aqitocrun, a treasure chamber in which he stores prized possessions.
 * Vanrakdoom: This sublevel was claimed by Lord Vanrak Moonstar for the followers of the Night Maiden in the Year of the Howling Moon. A sprawling shadow-cloaked temple of Shar and an army of creatures from the Plane of Shadow that defend the abbey dominate Vanrakdoom.
 * The Runestone Level: Not much is known about this small sub-level located between The Caverns of Ooze and The Terminus Level. It remains largly unexplored though rumors state it is linked to The Shaft by multiple small, twisting passages.
 * Shadowdusk Hold: This sub-level is among the deepest known sub-levels of Undermountain. It may or may not be connected to The Mad Wizard's Lair which lies just seaward of this sub-level.