Iuz is the half-fiend or cambion demigod son of Graz'zt, a demon lord of the Abyss, and the human witch Iggwilv. Originally described as very handsome, Iuz now appears either as an old man with poisonous spit, or a big, red, hulking demon like creature. His weapon of choice is the whip and greatsword. He is referred to as "The Old One" or Iuz the Evil, and worshiped as the God of Deceit, Pain, Oppression and Evil. He is considered the enemy of every sane and free person in the Flanaess and every religion, except his own and holds a particular hatred for Zagyg, Vecna and St. Cuthbert.
In 479 CY, a warlord in the Howling Hills left control of his land to his "son". By the end of his first year on the throne Iuz had assimilated the three surrounding fiefs. In less than three years, that territory was expanded, creating a small kingdom. Refugees from his conquests told tales of a road of skulls leading from the hills to the capital of the land, Dorakaa. They told tales of Iuz being the son of a demon and a necromancer, being a 7ft tall demon driven by destruction. They claimed he had an army of evil men, terrible wizards and savage orcs, and worst of all, fiends. It was about this time Iuz used a dark ritual to steal power and life force from many victims, making himself a demigod.
In 505 CY Iuz vanished. Many of the orcs, clans that had served him for years, as well as some of his human followers claimed he has ascended to full godhood, beginning to worship him, and actually able to perform cleric spells based on their worship. In reality, Iuz was trapped under Castle Greyhawk by the Lord Mayor of Greyhawk, Zagyg Yragerne in the Mad Archmage's own quest for godhood. During this time, some of his more opportunistic followers and a few of his fiends broke away to form the Horned Society.
Iuz was deceived and imprisoned in 581 by a powerful riven, Vecna. Absorbed into the powerful lich, Iuz served to fuel Vecna's ascension into a Greater Deity. Vecna was eventually thwarted by a group of adventurers, freeing Iuz in the process and allowing the Prince of Pain to renew his warring on Oerth. Iuz holds a deep hatred for Vecna as a result of his humiliation.
In 582 CY, Iuz appeared to the northern barbarian tribes as their imprisoned deity, Vatun, and using magic to control Sevvord Redbeard, ruler of Stonehold, used the combined forces launched an attack on Tenh. At the same time, Iuz's armies destroyed the Horned Society and conquered the Shield Lands, The Bandit Kingdom, the Barrens and the a portion of Furyondy near his border as well as the edge of the Vesve Forest boarding the Highvale. The war raged for three years before the various nations involved ended it. After the War Iuz seemed unstoppable. However, his control over the nomadic Rovers was loose at best and his control of the Bandit Kingdoms was challenged by various rebel groups within the land. Most of the regions under his control are monitored by the Greater and Lesser Boneheart, a group of spellcasters and clerics that serve as commanders of his forces and administrators of his empire, as well as the Boneshadow, his most valued spies.
In 586, Canon Hazen of Veluna used the Crook of Rao to banish many of the fiends from the Flanaess in what became known as the Flight of Fiends, which had a devastating effect on Iuz as it cost him nearly half his army Shortly after, Furyondy, Veluna and the Knights of Holy Shielding launched the Great Northern Crusade which saw the northern portion of Furyondy restored. However, the alliance discovered Iuz's forces had slaughtered every last man, woman and child, bringing them back as the undead. A small part of the Shield Land was reclaimed, though Iuz still controls the majority of the broken nation. King Belvor III of Furyondy then declared a permanent state of war against the Empire of Iuz, which would not end until the Old One was defeated and his evil wiped from the land.