"Nott 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."
source :
https://forgottenrealms.fandom.com/wiki/King_of_Shadows