The Ruins of Thossos

Excerpt from the official map of Thay
On the far western edge of the Plateau of Thay, well away from any major road or river, looms the ancient fortress of Thossos.  It began as a dark secret, swathed in concealing magics for a short time.  The Mulhorandi policy was to leave it alone as a cursed scar on the land.  Much later, the Red Wizards sent agents to explore the ruins and determine what should be done with them.  The good news was that they had some useful aspects.  The bad news was that the site was created too well for anyone's good and getting rid of it would be a monumental task.  

As far as anyone has been able to discover, the mighty complex was a last-ditch effort.  During the final days of the war between Narfell and Raumathar, Nar generals had pushed the boundaries between the empires to their limits.  They needed to gain a foothold close to Raumviran territory, but not so close that it would be immediately destroyed.  To get the most done as quickly as possible, Nar demonbinders made a deal to summon an entire fortress from within the Abyss.  

It was supposed to be a temporary stronghold from which an army of demons could be gated in.  No additional modifications were made to the land nearby and no records of its existence on any contemporary map have been found.  As native stone from the changeable Abyss, it was not visible without true seeing or having demonic ancestry.  Legends say that the central tower had a layer for each layer of the Abyss, spiraling up out of sight, though they seem too fanciful to be believed.  (Thossos is the name of the tower itself but there are arguments as to what it means, since both of its parts have multiple definitions.)

In the end, its insane scope didn't matter.  Raumviran battle-wizards discovered and assaulted the tower mercilessly, ripping it open and killing everyone inside.  The concealing effects fell, leaving the bizarre stones for all to view.  Few battle-wizards made it out of that fight and none were alive to search for it later when it began to reform.  Red Wizards theorize that the pact which drew it into the material world must have specified something grand.  Perhaps the fortress would remain until every trace of Raumathar was wiped from the face of Faerun.  In any event, the terms have not been fulfilled.  As far as anyone knows, the Red Wizards have not tried to complete them.

The fortified walls and outlying buildings are safe enough that guards are stationed there regularly to keep an eye on the surroundings.  It's usually considered a boring assignment.  The central tower, however, is off limits except to Red Wizards and their designated crews.  The top floors reshape themselves sporadically and the gates they possess to layers of the Abyss change, as well.  It seems that the tower has been trying to repair itself but is never quite satisfied.  Demons within cannot leave the structure without making a new bargain, but they can and will attack unwary visitors.  Ghosts of dead Nar demonbinders also remain, desperate to finish ancient business or to fight among themselves, blaming one another forever for failures that cannot be undone.

The underground layers are rumors, but they do exist, and they are not the result of the initial summoning.  As such, they are stable, but far from safe.  Local drow were called to the tower's presence as they ventured up to see what they could take.  They used its gateways well before Thayans claimed it (and there was a vicious fight over ownership of the premises which ended with Thayans allowing the drow access, with their approval and oversight).  The drow eventually herded burrowing creatures and slaves through to create a protective maze to the surface.  

Now it is a place where drow and Thayans can meet to discuss business.  All local drow factions know at least one way through the maze, as do the Red Wizards.  Sometimes slaves are traded, and it is said that no slave who has gone into the tower has ever returned.  Other times, it is the stage for longer term agreements, including ambassadorial trips in either direction.  Together, the drow and the Red Wizards stock and adjust the maze, changing the safe routes so that spies will have a harder time relying on them.

The Red Wizards send groups into the tower at intervals to clear out the upper floors, to search for spies or other meddlesome interlopers (since cultists have been discovered trying to get to their masters), and to see if any of the Nar ghosts can be resolved.  Teams search the mazes below for similar reasons.  Some groups return.  Others do not.  Receiving tower duty is an ambiguous assignment because it could be a way to make one's own pact with a demon, or to visit the Abyss, or to get rid of a rival.  Treachery between Thayans is often suspected but rarely investigated.  But it's also a way to vanish without a trace or lose your life in yet another secret project.  

Fierce but brief wars with creatures who seek to possess the tower have happened from time to time, as the guards outside know all too well.  So far, the worst invasions have not reached a population center.  The complex is a kind of open secret.  Thayans must show their papers upon approaching it as they must do everywhere else, but it is not as tightly restricted as the plateau upon which High Thay rests.  Guards will try to keep the curious away, but if they do get inside, it is assumed they will receive what they deserve.