Shayol Ghul (pronounced: SHAY-ol GHOOL) is a great black mountain, once an idyllic tropical island paradise and vacation spot, now the focal point of the Dark One’s power. It is located in the lifeless Blasted Lands, a place which bore the brunt of the War of Power. Within the mountain is the Pit of Doom, accessible only by a narrow stone passage where reality is wet clay to the Dark One; teeth of stone may brush at an intruder’s head or close on him entirely, or rise to give honor to a loyal servant of the Shadow.
The Blasted Lands are inaccessible, for they are past the Great Blight. Entire armies have died fighting the Blight, and it is all the forces of the Borderlands can do to hold it back.
The Pit of Doom itself is a great lake of lava, the only place where the Pattern is thin enough to make the Bore detectable. The sky seen here is not the sky seen from outside. This is where the Dreadlords swore oath to the Dark One. The True Power of the Dark One is most concentrated here; any who dared channel it would be instantly incinerated. The Dark One resides here, if the Pattern can be said to hold such a being. Here he can exert his will at full strength, even bending reality, and probability does to a degree. Any who channel there without permission are punished. Some things are only possible here; these include the mindtrap.
Below the mountain is the great valley of Thakan’dar, where shadow-forgers forge the tainted swords of Myrddraal at the cost of human souls. Lightning flashes up at the clouds in this place. There is no life, and it is as cold as the ice to the north, yet the place is dry as any desert. No man of the Light has ever seen Shayol Ghul and bore witness to the fact. Any who survived were turned or slain. Jain Farstrider was known to have tried, yet what he saw is unknown as he disappeared shortly after his return.
“I have seen Thakan’dar." Saying that hurt; the memories it brought were agony. He refused to whimper, forced the words out. “The great sea of fog, rolling and crashing against the black cliffs, the fires of the forges glowing red beneath, and lightning stabbing up into a sky fit to drive men mad.” He did not want to go on, but he made himself. “I have taken the path down to the belly of Shayol Ghul, down the long way with stones like fangs brushing my head, to the shore of a lake of fire and molten rock—” No, not again! "—that holds the Great Lord of the Dark in its endless depths. The heavens above Shayol Ghul are black at noon with his breath.”
— Padan Fain