The Skull was located about 50 million miles (80 million km) from the crystal sphere of Realmspace and was always in a stationary position. It was 12 miles (19 km) tall and 10 miles (16 km) wide and was shaped exactly like a human skull. The enormous structure appeared to be made of bone, but it quickly regenerated itself if damaged in any way. It also kept constantly refreshing its own air supply.
The hollow object was inhabited by a colony of mind flayers that secretly kept about 100 beholders of seven different subspecies as cattle. The beholders had their eye stalks constantly removed and their central eyes branded by their illithid owners in order to prevent them from using their abilities and to keep them blind. Since beholders and illithids had established collaborating efforts elsewhere, the existence of this experiment was a well-guarded secret.
The illithids used their beholder cattle to conduct breeding experiments in an attempt to develop their immense brains as trading commodities. After several generations of consuming beholder brains, by the mid-14th century DR the Skull's inhabitants developed the capability to levitate. This ability was viewed with envy by other illithids, but the Skull's breeders had incorporated them into their genetic makeup, so all their descendants from that point on were also capable of levitating.
Comets: K'Thoutek • King-Killer Star
Nebulae: Galleon Nebula • Color Spray Nebula
Far Realm-infested stars: Acamar • Caiphon • Delban • Gibbeth • Hadar • Khirad • Nihal • Zhudun
Other astronomical bodies: Caer Windlauer • Skull of the Void • Sargassos