Before humanoids ruled the world, aberrations warred for thousands of years with the dragons for control of Canus. During the war aberrations used powerful magic to create special prisons designed to hold dragons. These prisons needed to be beyond powerful to contain such mighty creatures.
The aberrations eventually lost the war with the dragons, abandoning their surface structures and retreating into The Underdark. Many of their prisons remain standing thousands of years later with magic and horrors untold.