Undermountain

Undermountain is a huge dungeon complex created by the mad wizard Halaster Blackcloak and located under the city of Waterdeep. It continually changes according to Halaster's strange whims. He summons monsters into his dungeon from their homes, trapping them there and creates traps to test adventurers venturing down into the dungeon, looking for fame and fortune.

Overview
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 and Kara-Tur from what is now the Plains of Purple Dust, a wasteland created by conflict with 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 evermore-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
 * -750DR: 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