The Weaver King was born Oleg en-Dal (later il-Dal), a brave nomad warrior who opposed the Stronghold. Gradually his tribe was killed off, and eventually he led his remaining people to the Stronghold to swear allegiance to the Evincar and start a new life in the City of Traitors. Most of the tribesmen were accepted but Oleg was imprisoned in the Stronghold dungeons and experimented on by Volrath, to pay for the soldiers he had killed. Months of mental abuse and solitary confinement ruined Oleg's mind, driving him mad.
When Volrath put a mother and her young child in a labyrinth with Oleg, he inadvertently killed the child and possessed the mother. Oleg experimented with his newfound powers of possession and control. "Oleg il-Dal died at last, and the Weaver King was born". When Volrath was overthrown by Crovax, the Weaver King was forgotten for many years. During the Rathi Overlay he was brought to Urborg along with the Stronghold and became a shadow creature.
The Weaver King spent the next 300 years after the Phyrexian Invasion creating a vast web of mental strands between the inhabitants of Urborg, a web which he traveled along like a spider, manipulating people and events for his own entertainment. Venser became his new favorite "toy", and he messed with many of Venser's plans until Lord Windgrace put a "Gladehunter mark" on Venser, which kept the King out of his mind.
Leshrac later found and manipulated the Weaver King toward his own ends.
One of his powers mentioned but never fully shown was his ability to dominate minds. He used this on Venser in the forest of Skyshroud once. But he had hinted at being able to take over a being's mind by slowly subduing their conscious mind. He also had the powers of telepathy and could access the minds of anyone he had left a thread in. He could cause damage to a being's mind with his silver threads much like one would grate cheese. It never said he could do physical damage to anyone except through controlling another being to do so. Most of the time he merely caused people to give in to their darker desires. He was known as a psychic vampire by Lord Windgrace and Jodah.
Venser eventually killed the Weaver King, by planeswalking while he held a pair of powerstones. The vast energy of the powerstones exploded outward, killing even the incorporeal Weaver King.