Watcher's Keep

Watcher's Keep was an ancient keep of unknown origin, that became associated with the deity Helm and the Knights of the Vigil. It was a powerful prison, used to house the most terrible beings whose power was troubling even to the gods themselves.

Location
It was located in the realm of Tethyr, west of the Wealdath near the coast.

Structure
The stone keep was built out of a rocky tor jutting out of the ground. It comprised six levels, in descending order from the uppermost floor.

The rooftop entrance to the keep was sealed by a hemi-spherical passageway. It could only be opened with the use of a unique wardstone. Along one side of the keep was a series of doorways that could only be opened from within the keep.

Interior
Each level of Watcher's Keep held a keyed portal that granted entrance to the next level.

The topmost level of Watcher's Keep served as a sort of temple to Helm. It held the private quarters of the keep's priests, a small Helmite tomb, as well as an extensive library that encircled a large central hallway. The hall was decorated with stone statues of various humanoid figures, built alongside four rows of pillared archways, each of which was adorned with a banner that depicted Helm's watchful eye. Within the center of the hall was the floor's portal; at its rear was the altar used by the members of the Vigil, set in front of an enormous statue of Helm.
 * Altar Level:

In order to activate the level's portal, the Helmite prayer ritual had to be performed at the altar in the correct manner. Doing so would activate the statues located in the central hallway, causing them to come alive with great hostility. One of these humanoid statues was equipped with the personal blade of the legendary Kara-Turan Yakuza named Usuno, while another wielded Foebane, one of the Baneblades of Demron.

The library held a number of notable tomes including the third appendix of Elminster's Ecologies, a golem manual, and fittingly, the History of the Imprisoned One.

As of the mid–14 century DR, this level was inhabited both by the ghost of Giltham the Archivist, as well as more malevolent spirits, such as vampiric wraiths and wandering horrors.

The second level of the keep comprised a central, circular room that was encircled by eight other, a library and laboratory designated for each of the four elements. The central room itself housed a magically enhanced cage and two portals, one that led to an exit outside the keep, and another connected to the keep's third level.
 * Elemental Level:

Within the center room was a magically-enchanted cage created by four archmages, each of whom experimented and studied one of the four elements. They were rivals with one another, often acting against the mage of their opposing element. Trapped within the cage was a chromatic demon, who had been tricked and imprisoned around the 4 century DR in order to secure the Imprisoned One on Toril. It could only be freed by using the four scepter keys in the possession of the archmages.

Each of the archmage's respective laboratories and libraries were quite dangerous. The air laboratory had a giant fan that created guardians of air when it was powered up; the slime chamber was filled with toxic mist, that formed as semi-sentient creatures; the air in the ice lab was extremely cold, to where it could freeze a creature solid; and the fire chamber contained a bubbling pool of lava, and granted its fire giant guardian a form of invulnerability. After the archmages were no longer present, the level became overrun with their creations; among these creatures were air elementals, mutated spiders, a giant snake, ice golems, and fire elementals.

The chromatic demon was in possession of the portal key that granted access to the third level of the keep.

The third level of Watcher's Keep comprised a dozen separate rooms, each connected to one another by portals. The rooms were oriented in such a way that portals were located at each of the cardinal directions, or along the north-south axis. To navigate the maze, an individual had to enter through the correct portals in a specific order – with the catch being that entering through a portal in which you already came, did not return you to your previous location.
 * Maze Level:

The different rooms were distinctly set as either wild magic or dead-magic zones, and were somewhat navigable by the central landmarks they featured. One room had a stone obelisk in the center, while another held a single tree. Yet one of the other rooms featured a stone pillar, within which was set Purifier, the sword of the lost knight Sir Wediyer.

The penultimate portal in the series led to cave set within natural rock formation. Within that cavern was the final entryway that offered passage to the keep's fourth level.

The predominant feature of the fourth level of Watcher's Keep was a device known as the machine of Lum the Mad, an icosahedral device that featured a number of mechanical, steam-powered arms. The operator of the machine was granted access its wide-range of powers, the scope of which was likely not fully known. By manipulating a series of colored dials, shaped buttons and levers of varying lengths, an individual could create a wide variety of effects, such as spawning beings from other planes of existence, conjuring fireballs and bolts of lightning, trapping the operator deep beneath the earth, or even increasing their mental acuity or physical strength. There were a total of 729 combinations that could be made with the various controls, though some of these gave duplicate effects.
 * Machine Level:

Just around the corner from the hall that housed Lum's machine was a smaller, secured room that could only be open by lighting a number of braziers with colored flames. Within this chamber was a mallet made of crystal, the only item known to be able to destroy the glass casing that protected the machine's operator. The chest with the crystal mallet was protected by a pair of minotaur guardians, Rock and Garock.

Beyond the machine room two areas that would later be taken over and reshaped by some of those creatures brought to Watcher's Keep from other planes.

The fifth level of Watcher's keep contained Helm's final seal, which kept the Imprisoned One bound to Toril and prevented it from returning to their home plane of existence. The seal could only be opened when three keys were used, one each for the mind, heart, and spirit.
 * Final Seal:

Down the level's west-facing hall was a puzzle-device featuring three pillars and colored globes that were dispensed by a machine. If the puzzle was not solved in the correct order various waves of murderous beasts, monstrous humanoids, mindless undead, and hostile spellcasters were conjured fourth. When the puzzle was completed, the mind key could be used on the final seal. Doing so summoned forth an aurumach and several ferrumach rilmani from the Outlands, all of whom were hostile to whomever turned the key.

The southern hall was watched over by the ghost of a knight of the Vigil. They presented three challenges that had to be completed in order to receive the heart key. The mental challenge involved several riddles posed by an imp, the perseverance challenge meant fighting an entire horde of orcs within a small cave, and the courage challenge included the defeat of a powerful green dragon. Using the key on the final seal summoned a powerful group of allies: the fiends Nalmissra, Xei Win Toh, and the Huntress, the marilith Y'tossi, the drow Ameralis Zauviir, along with their hive mother ally.

History
The keep was a millennium before the Era of Upheaval, around the 4 century DR, as a prison for the most dangerous and evil creatures the gods themselves wanted locked away.

While it was unknown exactly when, the archmages of the elemental level eventually killed one another in a massive magical explosion. The chromatic demon was left imprisoned within its elemental cage.

At some point a group of adventurers traveled into the keep in search for treasure and made their way down to the maze level, but were halted when one of the portals malfunctioned. Eventually most of the group died and the only one to remain was the elf who came to be known as Yakman. While he maintained possession of the staff of radiance that allowed entry to the next level, he became delusional after years of isolation, and even more fearful following the appearance the maze level's fiendish inhabitants.

Some time before the mid–14 century DR, a group of adventurers made their way into the keep and down to the fourth level of the Keep. The wizard Carston abandoned his group for the power of Lum's machine. After taking control of the device he managed to spawn forth various creatures from across the planes, some of whom fought one another for Carston's amusement. Among those relocated were the demons and devils that were engaged in battle in the Blood War, the particularly vain red dragon Saladrex, as well as numerous groups of illithids and githyanki warriors, some of whom had been traveling in their astral ship.

By means of Carston's blunders, the maze level became infested with the two factions of fiends who had been transported from their respective homes in the lower planes. Tahazzar led the group of devils, which included several cornugons, tieflings, and the pit fiend from the Nine Hells; while the Tanar'ri Ka'rashur led the glabrezu, succubi, and the balor that came from the Abyss. While stuck on the Prime Material plane, the two fiendish factions continued their their age-old conflict within the labyrinthine domain of Watcher's Keep.

The groups of illithids and githyanki each formed their respective encampments within the fourth level of the keep. Like the demons and devils, they continued their eternal battle while stuck on Toril, often for the amusement of the delusional Carston.

During the Time of Troubles, the Imprisoned One's seal became weakened by the Imprisoned One's power. While Helm was the only god not given mortal form in Faerûn, he directly tasked the Knights of the Vigil to watch over the keep, so as to never let the Imprisoned One escape or be slain on the Prime. Doing so would just sent the being back to its domain in the Abyss.

To keep the wards holding the Imprisoned One intact, the knights of the vigil were required to regularly sacrifice one of their own members. This tradition of duty continued until the, when the knights' leader Odren sought another means to preserve seal, and disregard the oath sword by his order.

Inhabitants
Eventually, however, the Knights of the Vigil lost their determination to sacrifice themselves, and began to spread rumors throughout the land to lure adventurers to the keep, promising rewards for helping them.

Gorion's Ward and companions came to the Keep and had to fight through five levels of the dungeon and withstand its traps and inhabitants. In the end, they met the Imprisoned One and had to decide whether to use a scroll given to them by the Knights to strengthen the wards and keep the demon imprisoned, and risk imprisonment themselves; or choose to fight and kill the demon, which would however free him from his imprisonment.

If they read the scroll, they were trapped, but the Imprisoned One, posing as an innocent kept prisoner by enemies of Helm, could help them escape. Once outside, the Knights of the Vigil ran to see what had happened to the prisoner, then one of their earlier members who sacrificed herself appeared and gave the Gorion's Ward a scroll from Helm, which would trap both the fallen knights and the Imprisoned One.

Inhabitants
The Imprisoned One was Demogorgon, the demon prince and ruler of 88th layer of the Abyss.

Reference
Wachende Festung