Mordeth was a mysterious man who inhabited Shadar Logoth.
Mordeth was originally a good man devoted to destroying the Shadow. He went in search of many powers, including the Finn. Unfortunately, he found a number of dark things, including the power of Mashadar, which would ultimately corrupt him and define the rest of his life.
Sometime during the Trolloc Wars, Mordeth came to the city of Aridhol. Quickly he gained the trust of King Balwen Mayel, who was only too happy to accept Mordeth's counsel. In his despair over the fate of his nation, King Balwen, also known as Balwen Ironhand, led the city's inhabitants to become increasingly zealous in their fight against the Shadow. They adopted a new battle cry to reflect the totality of their mission: "The victory of the Light is all".
Fall of Aridhol
Soon however, this zeal led to unfounded suspicions, plunging the population of Aridhol into fear and hate as citizens turned on each other. The city became something different than the Shadow, empowered by something other than the Dark One. Through the tutelage of Mordeth, Aridhol attempted to use the strategies of the Shadow to fight the Shadow.
It is said that the city was poisoned by the words of Mordeth, and that in attempting to serve the Light by using the tactics of the Shadow, they ultimately abandoned the Light.
Every single man, woman, and child and every living thing or remnant of life have since vanished from the once great city, leaving it a husk of its former glory. What exactly happened is unknown. It is theorized that the Mashadar entity that now inhabits the ruins, hungering for souls to devour, was the cause. Despite the existence of this fog-like killing machine, the only remaining trace of life ever to have existed within Aridhol (known forever afterward as Shadar Logoth) is the apparition of Mordeth. He somehow became trapped within the ruins of the city, and has been trying to lure adventurous souls to take even the smallest thing outside the city's bounds; an act that would set him loose upon the world once more.
Padan Fain was a peddler who became a Darkfriend in his twenties, for the promise of immortality. Sometime in 996 NE he was chosen to be The Dark One's Hound, tasked with seeking the Dragon Reborn. He made a dark pact with Ba'alzamon, who gave him dark powers to aid in his hunt, including the ability to track the three boys from Emond's Field: Rand al'Thor, Matrim Cauthon, and Perrin Aybara. After failing to kill the boys, Fain was forced to join the Trollocs in the hunt for them. When they hid in Shadar Logoth he managed to escape his Myrddraal overlords, only to be trapped by Mordeth. Padan Fain was the vessel Mordeth needed to escape the damned city; but because the Dark One had touched and changed Fain's soul, Mordeth could only merge with Fain, not overtake him.
The resulting entity was a rogue of the forces of the Shadow; not powerful enough to be a threat, but too powerful to control. After this merger, his compulsion to hunt Rand, Mat, and Perrin grew even stronger, aided by the fact that he could already sense where they were. He blamed the boys for what had happened to him. Following them to Fal Dara in Shienar, he was able to remove his ties to the Dark One, in fact making him more dangerous and evil than ever. In the chaos of the attack against Fal Dara, he escaped with Mat's dagger which he desired so badly, and the Horn of Valere. Sometime during this period he became far more powerful than the Myrddraal, often torturing them and forcing them to obey him against their will. It is said: "Myrddraal can make men sweat, but Fain makes Myrddraal sweat".
While traveling in the Ways, Fain was caught by Machin Shin, the Black Wind of the Ways that devours souls. The screaming voices of the Wind feared him, and while fleeing from him the Wind was next seen trying to force itself out of the Ways to devour Rand in Cairhien. This is a first, as it was previously assumed that the Wind could not leave the Ways, nor seek specific people.
Mordeth lured Rand and company to Seanchan-occupied Falme, where he lost the dagger and the Horn. Calling himself Ordeith ("wormwood" in the Old Tongue), he decided to forego force for manipulation, convincing Lord Captain Commander Pedron Niall of the Children of the Light and Elaida a'Roihan to oppose Rand. He also convinced Niall to send Whitecloaks to the Two Rivers, hoping it would draw Rand out. Unfortunately for him, he got a ta'veren he did not intend, and Lord Perrin Goldeneyes of the Two Rivers broke him of his Whitecloak affiliations.
At this time he is one of the most dangerous creatures in the known world, as dangerous as any Forsaken (though in far different ways). He can corrupt people simply by being in their presence, sense fear and tension like a hound, and can tell a Darkfriend on sight. He has also developed a way to inflict pain by touch with varying degrees of harm, a tool he uses to coerce and torture even Myrddraal into doing his bidding. Having recaptured Mat's dagger (or more accurately his dagger) from the White Tower, he finally feels complete.
He changed his name to Jeraal Mordeth, and sought out Toram Riatin, leader of the Cairhienen rebellion against Rand, whom he advised. When Rand showed up to neutralize the rebellion, Fain was able to wound him with the dagger from Shadar Logoth, which would have resulted in al'Thor's death, if not for the intervention of Aes Sedai and Asha'man. Mordeth then went to Far Madding with Toram Riatin, where, after dispatching renegade Asha'man, he lured Rand and Lan into a trap. The plot failed to kill the Dragon Reborn and the warder, leaving Toram dead and Mordeth injured.
A New Entity
He was still at large, and still hunting Rand. Robert Jordan has confirmed rumors suggesting that he may have been something entirely new, an entity or amalgamation never seen before in the Pattern. Able to create clouds and breezes of death in the blight, and tie the jumara into knots, he may have become one of the most dangerous entities in existence. He was killed by Mat in Last Battle.
Mordeth's name may be an allusion to Mordred, the bastard son and patricide of King Arthur. Alternatively it could simply be a contraction of "more death".