Deep Imaskar, also known as The City of the Great Seal, was created during the fall of the Imaskar Empire in −2481 DR by the wizard lord Ilphemon as a safe haven for his followers and family. Though it housed one of the largest populations of the Underdark, it was fully self-sufficient and isolationist, making an effort to hide its presence from the world. The city was located in the lower Underdark, somewhere below the Endless Waste. As of the Year of Lightning Storms, 1374 DR, it housed the largest group of surviving descendants of the Imaskar empire, a distinction it would keep until the founding and growth of High Imaskar sometime after the Spellplague.
In −2488 DR, Lord Ilphemon, along with a small retinue of servants and family, fled the destruction of Imaskar for the Underdark. The refugees would wander until −2481 DR, the year in which they founded the city of Deep Imaskar.  Ilphemon's descendants would rule Deep Imaskar until the year of −634 DR, when a cabal of necromancers would depose the king and slay the entire royal family. These same necromancers would rule until −511 DR, when a woman named Chaschara would overthrow them in a revolt.  After overthrowing these necromancers, Chaschara would create the title of Lady Protector of the Realm, creating the office of Apprehenders, Enactors and Planners in order to counterbalance their power.
By the year 799 DR, some plans for expansion into the Elemental Plane of Air were underway, but would be stopped permanently after several chichimecs managed to destroy their holdings in the plane, and then invade the city proper through the portal. After several days, once they were finally driven off, the position of Protector was abolished. The Lord Protector at the moment, Lord Styllofyr, was exiled from the City.
In the year 1370 DR, the Elder Evil Pandorym began to stir from its prison in the Palace of the Purple Emperor, extending its influence across the region of Vaelan. . A local wizard by the name of Ususi Manaallin escaped from Deep Imaskar in order to further her studies of the Celestial Nadir, an Imaskari demiplane, that same year. In 1374, Pandorym besieged the City of the Great Seal with a force it had amassed in the previous years, only to be stopped after Ususi found its prison and restored the seal. Settlers from Deep Imaskar would later go back to the surface in order to help found Skyclave, in High Imaskar. 
Deep Imaskar kept the customary Imaskari prominence of extradimensional spaces. The Deep Imaskari believed that a building not larger on the inside than it was on the outside was a waste of space; almost every building was full of such spaces. The area was not fully surveyed; local urban planners were known to change it continuously.
Important sites included the Purple Library  and the Enactor's Courtyard. The former was a library with the best sources on the Ancient Imaskari empire and history; the latter was the place where the Lady Enactor's retainers congregated, mostly to design structures and objects. Paradoxically the place was also a place of punishment, with criminals held in forcecages within the same space.
Deep Imaskar itself was built on a hollow of the Lowerdark shaped like a half pipe, 3 miles long, 1 mile wide, and a half mile high. Due to enchantments, gravity in the cave followed the curve of the ground, thus modestly extending the length of land available. Large towers converged into the center of the cave. Air was filtered, circulated, and refreshed through magical cloths hanging from the top of the cavern. The Great Seal was a magic circle 2000 feet across, drawn on the northern wall of the cave. It continuously radiated faint, penetrating light, which was visible all the way from the south wall of the city.  The Great Seal also had the magical ability to suggestion anyone in Faerun, or even in adjacent planes, that the Imaskari empire was well and truly gone.
As for 1372 DR, Deep Imaskar was ruled by its High Lord Planner, Illis Khendarhine, through the Body of Planners, who debated and decided on the policies for growth and development of the city. The Lord Apprehender and Lady Enacter oversaw information and resource gathering, and executed the planners' dictates respectively. Though the Lord Planner ruled alone, the Apprehenders and Enacters could block their decisions through inaction if they so chose.
In the Feast of the Moon of the 1372 DR Deep Imaskar had decided to partly break their Great Seal in order to send out scouts, spies, and explorers into the outside world, at the prompting of Lord Apprehender Ebrul Naramixna.
- Bruce R. Cordell, Gwendolyn F.M. Kestrel, Jeff Quick (October 2003). Underdark. (Wizards of the Coast), p. 138. ISBN 0-7869-3053-5.
- Richard Baker, Ed Bonny, Travis Stout (February 2005). Lost Empires of Faerûn. (Wizards of the Coast), p. 61. ISBN 0-7869-3654-1.
- Bruce R. Cordell, Gwendolyn F.M. Kestrel, Jeff Quick (October 2003). Underdark. (Wizards of the Coast), p. 139. ISBN 0-7869-3053-5.
- Bruce R. Cordell, Ed Greenwood, Chris Sims (August 2008). Forgotten Realms Campaign Guide. Edited by Jennifer Clarke Wilkes, et al. (Wizards of the Coast), pp. 138–139. ISBN 978-0-7869-4924-3.
- Bruce R. Cordell, Gwendolyn F.M. Kestrel, Jeff Quick (October 2003). Underdark. (Wizards of the Coast), p. 140. ISBN 0-7869-3053-5.
- Brian R. James, Ed Greenwood (September 2007). The Grand History of the Realms. Edited by Kim Mohan, Penny Williams. (Wizards of the Coast), p. 150. ISBN 978-0-7869-4731-7.
- Bruce R. Cordell (2006). Darkvision. (Wizards of the Coast). ISBN 0-7869-4017-4.
- Brian R. James, Ed Greenwood (September 2007). The Grand History of the Realms. Edited by Kim Mohan, Penny Williams. (Wizards of the Coast), p. 155. ISBN 978-0-7869-4731-7.
- Richard Baker, Ed Bonny, Travis Stout (February 2005). Lost Empires of Faerûn. (Wizards of the Coast), p. 64. ISBN 0-7869-3654-1.
- Bruce R. Cordell, Gwendolyn F.M. Kestrel, Jeff Quick (October 2003). Underdark. (Wizards of the Coast), p. 123. ISBN 0-7869-3053-5.
- Brian R. James, Ed Greenwood (September 2007). The Grand History of the Realms. Edited by Kim Mohan, Penny Williams. (Wizards of the Coast), p. 153. ISBN 978-0-7869-4731-7.
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