Morag Tovan is the capital city of the underdark, and can be found topographically somewhere beneath southern Lados. Abandoned by the drow when they made their exodus to the surface world in the Era of Exploration, the city lay dormant for some time. As the centuries fled, however, illithids from far and wide began to congregate to the city, inhabiting the cold stone and altering the structures to better fit their own needs. Now the city not only takes up the floor of the gargantuan cavern it was built in, but also crawls up the walls and even hangs from the ceiling, inverted towers and ziggurats dripping down like stalagmites.
The city, indeed the underdark as a whole, is now run by the Cult of Ilsensine, a pseudo-religious group of illithids who revere the Prophetess (which is what the illithid word "ilsensine" means). Their center of power lies within the Tower of Ilsensine, the largest structure in Morag Tovan, where they house the Scrolls of Prophecy. After a failed attempt at bringing about the Apocalypse, the End Times, during the Era of Heroes, the Cult has been largely silent from a surfacer perspective.
Morag Tovan is not a place for the meek or weak-willed. While outsiders are permitted into the city, it's alien laws and customs typically cause such visitors to meet their ends as food or slaves for the mind flayer citizens. If someone strong enough to survive visits, they must also be careful not to become embroiled in the sutble and deadly political games being played by the various Elder Brains throughout the city.
The capital of the underdark is no vacation spot, but to those with the strength and savvy to navigate its myriad dangers it can reveal secrets both wonderful and terrible. Goods and services outlawed by even the most liberal states on the surface can be found in abundance in Morag Tovan. To say there is a thriving black market of poisons, extraplanar goods, and slave flesh would be to say that the sea is wet. Thanks to the psionic nature of its inhabitants, sensory experiences unavailable anywhere else can be found for those jaded with what is normally available. But such thrill seekers should heed the old warning and be careful what they wish for. They may end up biting off more than they can chew.