The King of Shadows (originally known only as the Guardian) was a dark creature who was both born of the Illefarn empire, as well as the reason for its destruction.
Not much is known about the King's early days, save that he was a talented human magic user who was trained for twenty years by the elf Master Isym in the arcane arts. Fiercely patriotic to his homeland, the Second Illefarn empire, the King was one of the hundred who answered the call by the priest Annaeus to sacrifice himself to become a weapon against Illefarn's enemies: the orc tribes to the north, and Netheril to the east. After he was selected out of all the candidates, the King underwent a 100-day long, extremely painful ritual in which Annaeus merged the man with the Weave. When the ritual had passed, the man had lost all traces of his former self; he was now known as "the Guardian," a perfect weapon with only two goals: to protect Illefarn and destroy its enemies.
Fearing this Guardian, even Netheril dared not attack Illefarn, and so the empires co-existed for a time. However, when the wizard Karsus killed Mystryl, then-goddess of magic, via the spell Karsus's avatar, all magic ceased to function for a time. This caused the tie between the Weave and the Guardian to be shattered, and Netheril's floating cities to fall from the skies. Even after the goddess had been resurrected as Mystra, the King was unable to draw upon his old power. Therefore, he turned to the Shadow Weave, a dark magic source governed by Shar that turned him into a creature of darkness. Some Illefarn citizens came to fear this new incarnation of their Guardian, and sought to kill him before he could turn on his allies, even though he had made no attempt to do so. The six Silken Sisters attacked him in a forest, but were severely beaten. When they returned from the woods it was discovered that the King of Shadows had magically fused the six into one tortured creature. Deciding action had to be taken, Annaeus devised a Ritual of Purification, and assembled a large group of wizards and priests to attack the King of Shadows. Though they were successful in completing the ritual (apparently wounding him), the group was beaten when some of the Guardian's old friends decided to attempt to reason with him instead of slaying him. The King of Shadows was finally defeated when the rest of Illefarn's forces attacked the King in a later battle. Many soldiers, and the great wyrm Nolalothcaragascint, died in the battle, but a group of wizards was able to successfully seal the King away in the Plane of Shadows.
Centuries later, the King of Shadows threatened to return from the Shadow Plane to carry out his ancient purpose of the defense of Illefarn. However, Illefarn had long since fallen, its once proud cities reduced to foreboding arcane ruins, and the mighty citadel of Rivenguard Keep, where he was chosen to become the Guardian, was now overrun by ogres and goblins. The King of Shadows, however, held to two points of logic that though seeming twisted, were actually very well thought out when one analyzes his nature and base purpose: firstly, many of Illefarn's ancient foes remained; secondly, the King of Shadows reasoned that Illefarn's descendants were an extension of the empire, and thus its very legacy, and therefore it was his duty to protect them. In order to carry out these aims, the King of Shadows began to tear a portal from the Shadow Plane to the Astral Plane, where the native githyanki attacked him. Though the King was powerful beyond all mortal comprehension, the silver blades of the githyanki inflicted considerable harm to his form, though never enough to kill him: his outer skin of the Guardian lingered, protecting the essence that had to be struck to undo him. The githyanki Lich Queen Vlaakith was informed by Nolaloth of the King's original nature, and the wyrm suggested that they attack the portal itself.
After the Githyanki drove him from the Astral Plane, the King of Shadow was still able to somehow return to Faerûn, and the Sword Coast North, where he began to extend his influence from the Mere of Dead Men.
In his many travels across the planes and lands of the Realms, the powerful warlock Ammon Jerro, a mage whose power rivaled or exceeded the likes of Elminster Aumar and Khelben Arunsun, learned of the King of Shadows. Quickly comprehending the threat he posed to the lands of Faerûn, Jerro committed himself to using his considerable arcane might and connections to darkness to vanquish the King of Shadows and save the realms. He traveled across the planes and lands where the Fallen Guardian did battle, and through his infernal contacts and the ancient crystal wyrm Nolaloth, the Warlock learned that githyanki silver swords were the only weapons capable of coming close to killing the King. He then learned that the most powerful silver sword, a god-worthy silver blade once held by the githyanki hero Gith herself, was the only weapon that could utterly destroy the Guardian of Illefarn.
He went to search for the Blade in the place where Gith was reported to have gone missing, the Nine Hells. There, Jerro was able to retrieve the blade from Levistus, the Still Lord of the Fifth Circle of Hell, and his servant Mephasm. He then fought the King in a titanic battle in the tiny village of West Harbor, where he banished the King. The Blade was shattered in the fight, however - blasting Jerro himself into the Hells - and one shard was embedded in the chest of the infant who would grow to bring about the King's final destruction.