Hellgate Keep

Hellgate Keep, formerly Ascalhorn, was a city of evil on the northeast edge of the High Forest. It was destroyed in 1369 DR when the Harpers used the Gatekeeper's Crystal, and is now overseen by the treant Turlang and his fellows.

Interior
The audience chamber stood high, with a vaulted peak ceiling that had a broad hole roughly patched by rubble, and was largely empty. Hovering beneath that hole was an inverted purple pyramid, exactly  per side, that illuminated the area with pulsing magical energy. It came into existence following the use of the Gatekeeper's Crystal on the Keep. The pyramid absorbed all magic that came in contact with it and nonliving items that weren't relics or artifacts would potentially be disintegrated.

In each corner of the audience chamber stood four cauldrons of oil, situated behind huge oil lamps. On the eastern half of the room were a series of cages, long and  wide, that were so short that prisoners were to rest on their knees.

In the center of the audience chamber stood an eight-step dais, long and  tall, topped with a throne made from the bones of a dracolich. Whoever sat in it was protected by a globe of invulnerability. There were also three steel rings in the dais, meant for chaining prisoners that brought before Grintharke and the Keep's later demon rulers.

Ascal's Horn
The fortress town of Ascal's Horn was founded in the by the Eaerlanni elves in the northeastern corner of the High Forest on a peak known as Ascal's Horn. The construction efforts were led by one Ascal Rachiilstar and its walls were warded with what many considered to be near-mythals. Ascalhorn was generally thought to have been built by the Eaerlanni so as to protect the boundaries of their kingdom from the area's orc hordes. Though other scholars speculated that it may have been a preemptive defense against the rising human nation of Netheril.

In the, following the fall of the Netheril empire six years prior, the Eaerlanni elves took pity on the Netherese refugees and sheltered them.

Ascalhorn
Thirty years following their admittance to the town, the human population had grown to become the majority, taking up most positions of office. Its name had been shortened to "Ascalhorn" and it was renowned as a center for magic, where human wizards could learn the magic of the elves and Mystra.

The Eaerlanni elves had given it over to the humans under two conditions. The first was that they had to help defend the kingdom, for they felt the humans were better fit for dealing with guarding against the orc hordes, while the second was that they had to abandon the use of Netherese magic Those who defied the the second law faced the death penalty.

When the city of Myth Drannor rose in 261 DR, Ascalhorn one of the first cities to ally with it and through this alliance their arcane nature grew even greater. Around this time they also cultivated close relations with Silverymoon. However, much of the arcane knowledge in Ascalhorn was hoarded by the elitist, paranoid mages and wizards.

Following the fall of Myth Drannor in 714 DR, many refugees fled to the city of Ascalhorn. They were generally accepted by the populace, but the magic elite of Ascarlhorn grew ever more paranoid and fearful of losing their positions in society.

One such wizard was the Netherese arcanist Wulgreth, whose concern for power led him to summon baatezu to the city in 820 DR. But Wulgreth could never fully control them and over time the baatezu steered Ascalhorn's wizards into more covert and open warfare with one another. It escalated to the point that they fought in the streets twice every tenday. The other citizenry, uncomfortable from the baatezu's unconscious influences and the growing fighting between the arcane elite, gradually emigrated from Ascalhorn.

Fall of Ascalhorn
The baatezu's grew ever bolder over time until they eventually convinced their masters to accept lichdom. Through them they openly dominated Ascalhorn. In an act of desperation the few remaining wizards, under the guidance of fey'ri, summoned demons to the city in 880 DR to combat them.

Within two years the demons were triumphant, though most life in the fortress had died in the process and demons claimed the area as their territory. The balor Grintharke would bring over more of his kin and ally with the nearby orcs to wage war against the kingdoms of Eaerlann and Ammarindar, bringing them to ruin as well.

Maerstar, a bard and one of the few humans who had escape the massacre that occurred at Ascarlhorn, would write a song in 882 DR that rechristened the fortress as Hellgate Keep.

Hellgate
In 883 DR, Wulgreth finally fled the fortress and took refuge in the ruins of Karse.

In 886 DR, Elminster and Khelben Arunsun used knowledge from a collection of scrolls written by mythal designer Mythanthar in conjunction with the ancient wards the Eaerlanni architects placed in Hellgate Keep to construct a near-mythal ward that would prevent the demons from using their gate abilities. Their wards were placed around the keep by members of the Harpers.

In 890 DR, the demons began attempting to tunnel beneath the wards, in the process connecting many of Hellgate's cellars to its sewers. In 898 DR, some of the first tunnels beneath the sewer are opened up, exposing ancient hidden crypts with magic that the demons plunder. By 912 DR, the demons' tunneling efforts had brought them all the way to the Nether Mountains.

In 919 DR, the demons are forced to abandon their deeper tunnels due to the Morueme clan of blue dragons. Eventually they resumed their tunneling efforts and in 1221 DR had managed to connect with the deep tunnels of fallen Ammarindar and later the Nameless Dungeon in 1356 DR.

In 1365 DR, Grintharke faced seven Evereskan elves his forces had caught spying on the fortress a year prior in a gladiatorial arena for sport. Using the artifact Shattering Swords of Coronal Ynloeth, the elves managed to slay grintharke, losing three of their own in the process, and then killed his vrock lieutenant before dying themselves. The other demons flew into a frenzy of in-fighting, each wanting to assume control over the keep, and by the end of the day only seven of the greater and true tanar'ri remained. The most senior officers among them - the mariliths Ssaarn, Mulvassyss, and Amassyra - agreed to a truce and formed a triumvirate. But a year later, Amassyra was assassinated by her two partners.