Abrynthel-Kast

To the east of the Underdark's Cackling Sea is a region where the influence of the Far Realm begins to bleed in to the Prime Material Plane. The laws of nature are fickle here, and often genuflect to the un-laws of the Far Realm. This area is considered one of the most dangerous places in the Dark Below, hazardous to both your body, and sanity. Called Abrynthel-Kast, this region is home to aberrations of all kinds, who themselves are antithetical to the material plane. There is very little known for sure about this twisted realm, other than that it was once home to a great Illithid capital. The nature of that colony, and its current status, is unknown.
The name of this region is an ancient elven dialect that translates roughly to The Thinning of the Veil. It was named such by explorers from the undead empire of Lachrymose to the far east.

The Expedition of the Heroes of Silverpass Knights of the Argent Order

The party descended into the Underdark to confront the spider king Helmogog after his brood emerged on the surface and began to attack cities and villages in the West Realm. After traveling across the Cackling Deep, the party turned towards Abrynthel-Kast in an effort to save time in their journey. They have discovered the following points of interest in the ever-shifting strangeness of the Kast:

• The Mire of Memory - a series of shallow streams and creeks whose waters appeared to possess some kind of psychic hallucinatory properties. Sentient creatures that entered the Mire had select memories visualized in the fog, visible to other creatures. Bastien Selar'wyn identified this location after successfully communing with some kind of intelligence that manifested as writhing roots and tendrils along cavern walls. While here, the party discovered a chamber that housed a bizarre mirror-like creature and Faceless Wanderers, whom the party had encountered at the bottom of the Deepspire Passage.
• The Grotto - A seemingly serene conflux of subterranean waterways that housed numerous cave flora. The party was able to camp at this location, despite a pack of vile, tumor-ridden humanoids flocking through the area.
• The Giant's Prison - at the end of tight corridors and tunnels the party discovered a large chamber that housed a number of Stone Giants who found themselves trapped here after they fell into a sinkhole. This chamber was heavily suffused in faezress, and appeared to have warped and twisted the minds and bodies of the giants. One giant in particular appeared to have been granted eldritch powers while in this prison, and claimed to have served the god Tharizdun.
• The Cyst of Xyryxyr - The party entered into a large, artificial, nearly spherical cavern with a massive stone column at its core. The barren, rocky landscape had an anomalous spatial effect on the party, making travel in any direction but towards the tower impossible. They found an ally in a strange, lanky, cyclopean humanoid known as I/Am, who informed the party that they would be trapped here unless they could ascend the tower and deactivate an artifact called the Dreamcatcher, a bizarre magical device created by the mad "mage" Xyryxyr the Insomniac.