Gagethga’s lair is the demiplane known as the Halls of Silence, an eerie labyrinth of chambers, halls, and vaults made from mirrored crystal. The mirror-perfect surfaces of the lair periodically shatter soundlessly and slowly reform as if melting back together.
Gagethga is the name of the most psionically potent daelkyr sealed in Khyber. Known as the Master of Silence and the Lord of Shattered Thoughts, Gagethga exudes an aura of madness more potent than other daelkyr, as the aura carries with it the overwhelming weight of Gagethga’s psionic might which constantly seeps from its mind like water from a dam about to burst.
The Silent Master. The appearance of a daelkyr differs from one viewer to the next, but by all accounts they appear to be creatures of picturesque perfection. They are flawlessly beautiful, except for the flaws they choose for themselves. The Master of Silence usually appears as a statuesque Adonis of a man, with long, flowing raven hair and piercing eyes the color of the bluest ocean. Where its mouth would be, however, is a smooth, featureless expanse of alabaster skin. It is unclear if Gagethga even possesses an actual mouth or working jaw behind its missing lips. Because of its complete lack of a mouth, Gagethga obviously cannot speak. It is said all sound dies in the presence of the Master of Silence.
Shards of the Mind. The Lord of Shattered Thoughts crafts psychosis and delirium like its peers craft flesh. The most terrifying tales of Gagethga tell of its ability to cleave the neurosis it itself creates in a person’s mind by shattering their psyches, then manifesting those malignant thoughts into physical form, creating a Unknown. As the preeminent lord of telepathy among the daelkyr, Gagethga’s telepathic mind can extend for miles, beyond even the range of an illithid elder brain.
The Silent Cults. Gagethga has many cults among the backwater tribes of the Shadow Marches, and it gifts its cultists with psionic potential. Mind flayers, cerebriliths, and other psionic aberrations and fiends are revered as the divine messengers of The Lord of Shattered Thoughts, and elder brains are believed by many to be living altars to Gagethga’s divinity.
Due to recent events, the Master of Silence has discovered the psionic potential within kalashtar and the Inspired. By examining the minds of kalashtar it has broken, Gagethga has discovered the existence of Adar in Sarlona, a land where psionic practices are widespread. The Lord of Shattered thoughts has begun expanding its realm westward, searching the paths of Khyber for access underneath the mountainous refuge of Adar. Should the Master of Silence find its way to Sarlona, a psionic war of epic proportions may consume the entire continent.
Source: Psion's Primer