In The Wheel of Time
, a mountain in the Blasted Lands
, surrounded and protected the the Great Blight
, Shayol Ghul is NOT where Dark One is imprisoned.
Dark One is, technically, sealed where the sun doesn't shine. Shayol Ghul, and especially its Pit of Doom along with lake of fire and all, is a place where so-called "thinness" exists in the Pattern, making it the place most easily affectable to Dark One. Because of this, Shayol Ghul would be highly dangerous place for a follower of Light even without the Shadowspawn surrounding it and everything; Dark One himself could, frankly, blast the shit out of you there.
The location of Shayol Ghul is where used to be, in the Age of Legends, a beautiful island, even a paradise, surrounded by sea. It was popular vacation target, but it also had some native population and apparently a center of research also.
At the very end of Age of Legends, there was a very beautiful, smart and ambitious woman, very strong in One Power, called Mierin (latter to be known as Lanfear). In her experiments on finding the source of One Power that doesn't depend on which sex you are, she stumbled on something interesting. She somehow managed to "bore" (symbolically, obviously) a hole in it; this thing was to be known, surprisingly, as the Bore. By doing this, she set the Dark One free, bring evil back to the world in its fullness, ending Age of Legends, starting War of the Shadow and eventually Breaking of the World.
Shayol Ghul was wrecked, ruined and being the center of Dark One's influence, became the center from which the forces of Shadow could begin their advance in conquering the lands of Light. In the end of the War of Power (also known as the War of Shadow), when all hope seemed gone for Light, a plan was devised by the Aes Sedai, Lews Therin Telamon being the chief advocator of this plan. Idea was, that they would build 7 seal made of cuendillar, Travel to the Shayol Ghul and place these seals so that Dark One would be bound once again.
The plan wasn't agreed on by everyone; it was suspected that a little mistake in placing these seals might rip the prison so that Dark One would become completely free. It was considered too risky, and another faction, led by Latra Posae Decume, suggested that they should rather build two gigantic sa'angreal and use these to seal the Dark One. As the squabbling continued, both plans advanced: the 7 seals were made, and so were the 2 sa'angreal. In disagreement, the Tower was divided, most female channelers of sufficient strength in One Power siding with Latra Posae. It was thought that Lews Therin could not fulfill his plan without aid of female channelers.
However, these sa'angreal required certain ter'angreal to be used, as the force of those sa'angreal was too great to be accessed directly. Upon making these ter'angreal, Shadow's forces managed to make another great advance, conquering the city where these ter'angreal were manufactured. These ter'angreal were tried to smuggle out, unsucceendingly, and Light could not access their sa'angreal without them.
In all this, LTT grew inpatient, as Latra Posae's faction still refused to consider a direct strike to Shayol Ghul. He collected a group of strong, young and rash male channelers, known as the Hundred Companions (although they did number 113), and 10000 soldiers in secret. Then, he jumped with his fellows, without asking permission, and struck at Shayol Ghul.
Their strike did succeed, and Dark One was sealed again. For some reason, possibly a conference of the Shadow, all 13 Forsaken were at Shayol Ghul at the same time, so LTT's strike did manage to decapitate the whole Shadow's leading army at one strike. This would have been a jolly good thing, especially with some of the heroes surviving-68 and LTT, to be exact-but Dark One's counterstroke succeeded as well: he tainted the saidin, causing all males channeling One Power to go mad. This began Breaking of the World that wrecked the world and destroyed most of the civilization. Still, it is better what would have happened without...
--the information about strike adapted from a short novel by Robert Jordan, the Strike at Shayol Ghul, available at www.tor.com/shayol.html