The kingdom of shadow - Richard A. Knaak

After a lifetime of research and calculation, the Vizerei sorcerer Quov Tsin, thinks he has figured out the secret behind the ruins of the lost city of Ureh. He hires a pack of mercenaries from the West and they set out to search for the wealth of the fabled city - the riches and knowledge that disappeared without a trace along with the people of the ancient city, where mighty sorcerers once sought to gain entrance to Heaven itself. His calculations prove correct and phantoms soon begin to appear in the darkened city. Before long, the ruins return to life, but not as the Mecca they imagined. Instead, as the adventurers become a part of the magical life of the city, they come under the attention of its ruler, Juris Khan. Members of the party begin to disappear. Soon Tsin and the necromancer Zayl begin to suspect the horrible truth, that the quest for Heaven led the ancient sorcerers, and their city, not to Heaven but to Hell...and thanks to their own interference, no barrier will bar the possesed denizens way back into the natural world.