This unnatural geographical feature is shunned by all save the most fearlessly evil. A great pathway of granite slabs, up to 25′ wide and 12′ high, descends in a perfectly orderly formation down to the sea from a half-mile inland, with the causeway heading to the Isle of Cursed Souls (known to some as the Isle of Lost Souls). Extraordinarily, this causeway is never submerged by the tides, even though seawater might stand 80′ or more high either side of it shortly before it rises to the shoreline of the magical Isle. The whole causeway radiates intense evil, and magic, if such is detected for.
During the fullness of Celene, fiends of many kinds stalk the causeway. Tanar’ri and baatezu rend at each other, tearing each other apart. They gleefully attack anything foolish enough to approach within a half-mile or so of the causeway. The fiends appear to be bound to that distance, however, and cannot travel farther inland. Usually, but a handful of fiends will appear at full moon. Very rarely, perhaps once every 80 years or so, countless numbers of lemures, manes, dretches, and least fiends of all kinds will appear as great legions driven on by a few greater fiends in an orgy of mindless slaughter and destruction.
Stone fragments from the causeway have, rarely, been taken and enchanted by men of great evil to craft dark magical artifacts. This is surpassingly dangerous. One tiny slip in a process which might take dozens of spells and months of time will leave the enchanter helpless in the face of a gate opening and a powerful fiend emerging, enraged, to slay him. Some of the oldest books of Flanaess Oeridian mages give riddles and allusions to the work of Ur-Flannae mystics with this ghastly substrate. But such artifacts are lost to Oerth, perhaps. In more modern times, only Delglath of Rinloru is known to have crafted any items from the stone of this atrocious place. Even masters of the dark arts such as Xaene and Karoolck would hesitate to follow his example.
Across the causeway lies the Isle of Cursed Souls, which is avoided by all sea vessels. In a radius of roughly 10 miles, it is said that the very rocks of the sea bed will rise and hole any vessel approaching more closely. Further, any lost at sea will become bound to the isle as ghosts, tormented by unknowable horrors for eternity. Sailors will jump overboard into stormy seas with a prayer to Procan for their souls rather than enter these waters. Intelligent sea dwellers such as sea elves, selkie, or dolphins warn ships away from the area.
The isle itself appears only to have the ruins of an ancient monastery or mansion house atop its sheer cliffs, but distant scrying reveals an intensity of evil and magic beneath that site which is powerful enough to threaten insanity to the diviner studying the place. In texts that are now no more than a rumor in the night between old sages, the place is said to be the last resting place of three of the greatest of the Ur-Flannae. If this is true, the perils below it do not bear even thinking about.