Yug'Voril

Yug'Voril is a city in lowest depths of the Underdark. The city is thousands of years old, implying that it pre-dates the Calamity and possibly the Age of Arcanum. The city is most recently occupied by illithids and their thralls, but the city's original builders disappeared long ago, and according to Clarota, no one knew who originally constructed Yug'Voril. His people found it abandoned generations before Vox Machina journeyed into the Underdark.

City description
Yug'Voril is located inside a large cavern on an island surrounded by a lake. The walls of the cavern are lined with veins of faintly—almost uselessly—magical blue crystal, and stretch far up into the darkness well beyond the range of sight. The shore is comprised of a type of gravel very similar to sand. The water surrounding the island is fresh water and clear enough for one to see into the lake provided that the person has light. The cavern is very humid, and the temperature is cold.

The island itself is mostly covered with a forest of glowing, bioluminescent, tree-like mushrooms, and the geography of the landmass is made up of eroded areas where cliffs have formed. There are also a couple of smaller islands off its coast with ruins from Yug'Voril's former construction. The only land crossing to the island is a large, greenish, jade and marble vein-like stone bridge, although at some point, creatures (most likely the duergar) used ships to traverse the lake.

Hundreds of illithids occupy the city on the island. Like the bridge, the buildings are made out of vein-like jade and marble stone, and none of the buildings are perfectly straight, as everything has a slight non-euclidean curve to it. Because the buildings have decayed with time, the illithids have slowly rebuilt the center part of their city as the population and number of thralls they possess has increased. Green spores from the mushroom forest light up the city. A large pseudo-pyramid with a flattened, open top stands in the center of the city.

Additional Info
The illithids use the fungus that grows in the mushroom forest to feed their thralls.

Notable locations

 * The temple: Located in the center of Yug'Voril where the elder brain resides; served as K'Varn's fortress.
 * Rift to the Far Realm: On one side of the cavern is a tunnel, 15 or 20 feet wide, in which the veil between the Material Plane and the Far Realm is thin. This is where Clarota believes K'Varn entered the cavern.

Demographics

 * Dwarves
 * Intellect devourers
 * K'Varn's abominations (possibly once Vox Machina escaped)
 * Mind flayers
 * Ogres
 * Trolls

Creatures
The cavern and island are also home to other creatures. Within Yug'Voril, the illithids have also enslaved dozens if not hundreds of creatures. However, Vox Machina could not determine the various species that the mind flayers kept deep in the Underdark. The elder brain also controls many of the creatures that live on the island.


 * Bats
 * Cavern insects
 * Cloakers
 * Fomorians
 * Gricks
 * Illithids
 * Small sightless rats
 * Ghouls
 * Wandering spores

After successfully rescuing Grog from Queen Ulara, Vox Machina and company traveled deeper underground until they reached Yug'Voril. Using a combination of magic and brute strength, the party was able to lift a sunken ship out of the water and let Percy repair it. With Keyleth in shark form towing the boat, the group slowly made their way around the island, slaying monsters and looting along the way.

Eventually, after coming to a rest near where K'Varn had entered the Material Plane, they learned K'Varn's identity as a beholder and that he also possessed a Horn of Orcus. They then planned their attack strategy and made for the main island. While attempting to set up a temporary base in a nearby abandoned village, they encountered a fomorian giant, which they eventually managed to mind control and have him serve them.

In order to defeat the beholder, the party dropped the fomorian giant on top of a giant crystal structure that had been built to control the elder brain, killing the giant in the process. While in combat, Tiberius was too terrified to fight the beholder, and instead went to enlist the newly freed illithids to join them in the fight. During the fight, Lady Kima was turned to stone by one of K'Varn's eye rays, and later on, Grog was killed by another. However, Pike was able to revivify him before he died for good. Eventually, Vox Machina managed to kill K'Varn with a combination of repetitive damage, quick healing, and lucky saving throws.

After K'Varn fell to Vex'ahlia's attack, the party rejoiced until they noticed the Horn of Orcus bringing him back as an undead. This prodded Tiberius into action, and after a failed attempt and some additional damage, he succeeded in ripping the horn from K'Varn's forehead. This ended K'Varn's life, though the horn still holds all of its original power.

Afterwards, Clarota and the illithids betrayed the group and attempted to kill and/or feed on the party members. The group was able to escape with the Horn of Orcus, doing significant damage to the elder brain in the process. Percy was also able to kill Clarota while the latter was attempting to eat Scanlan's brain. The party made their way to the far side of the island where Tiberius quickly created a teleportation circle to send them to Emon.

Aftermath
The temple was slightly damaged during the battle with K'Varn and Vox Machina's escape. The elder brain itself was nearly ripped in half, and would take time to heal. That was one of multiple reasons Lady Kima doubted that the illithid colony would continue its drive toward the surface.

Trivia

 * During the Calamity, the Dawnfather and the Everlight banished the Crawling King to a "sliver of the Far Realm that now borders the deepest pits of the Underdark, where the boundaries between the worlds grow thin and birth terrible abominations bent on subjugation."