The Thayans do not possess a monopoly on demonology and evil magic. Thay was once a province of the Mulhorandi empire, which was an ancient and decadent culture that has only recently begun to reclaim some parts of its former glory. Followers of the Mulhorandi gods Set and Sebek are guilty of atrocities that would make a Red Wizard turn pale. The Mulhorandi themselves were once subjects of the Imaskari Empire, an extraplanar race ruled by powerful wizard-kings.
With a history so rich and dark, it is little wonder that many forgotten portals lie hidden in the deserts and mountains of Mulhrond.
One such portal is in Raurin, a region frequently called the Dust Desert. It was here that, ages ago, the Mulhorandi waged their war against their Imaskari masters. The land was permanently scarred by their conflict, and it is now nearly uninhabitable. Among those few who brave the perils of the Dust Desert are the worshipers of Set, who maintain a temple in the region. The secluded, inhospitable geography provides the perfect refuge for the depraved worship of their ophidian god.
Unknown to the worshippers of Set, hidden perhaps even from the yellow eyes of the serpent-god himself, a long-forgotten portal to an Outer Plane is buried beneath the sands in a nearby ruin. Occasionally, the desert winds shift and expose the worn stones of this ruin, sometimes allowing the angular top of the portal's structure to be seen in the light of day. This is only a momentary phenomenon, however, because the desert sands shift quickly, as though guided by some unseen hand, burying the ruins and all those unfortunate enough to be standing near them.
Should someone reach the portal through the constantly shifting sand, they discover that it will not open readily. The portal is keyed to allow only divine spellcasters who have chosen the Evil or Darkness domain. The spellcaster in question must cast a spell from either domain at the portal, which then absorbs the spell's power. Once activated, the portal remains active for 1 round. When active, its surface seems like a rippling sheet of darkness, giving no indication of what may await on the other side.
The Outer Plane that the portal connects to is a lonely, dry place that seems to resemble the Dust Desert -- except the sun remains at perpetual twilight, and an unforgiving cold grips the landscape. In this grey, dry land no native creatures live. Characters who pass through the portal find that they can breathe normally, but they are hard-pressed to find anything to eat or drink. Characters passing through this portal have a final, terrible surprise in store for them. The portal on the Outer Plane is malfunctioning, and the characters cannot return to their original destination. Malfunctioning portals are described in the Forgotten Realms Campaign Setting.
This nameless Outer Plane, while home to no living creature, may yet contain artifacts of the ancient Imaskari -- or perhaps some other, still older magic that would perhaps be better left untouched.
How to Incorporate the Mulhorandi Portal Into Your Campaign