The Crook of Rao is
In Isle of the Ape, the Crook is described as a crooked mace (“a mere toy, it seemed,” according to Tenser), but in From the Ashes and Dragon #294, it is described as a crooked staff like a shepherd’s crook. It is carved from hornwood, with jacinth, alexandrite, sard, topaz, and other precious stones embedded in it. Its shaft and headpiece are formed from cold iron and silver. It gives the impression of immense age.
Powers ascribed to the Crook of Rao include the ability to turn evil beings of the lower planes as clerics do undead, banishing them to their infernal homes for a hundred years. In addition, the Crook prevented such beings from summoning others of their kind within a mile of its presence. Less impressively, it could also be used as a weapon, helping to deflect fiendish attacks as well as banishing them on a successful hit. There were no legends of the Crook functioning as impressively as it did in the hands of Canon Hazen of Veluna in 586 CY, though there are now.
According to myth, the Crook of Rao was first brought to Oerth by the deva Incarum, who used it to banish the minions of Tharizdun from the face of the world. Its purpose complete, the artifact was lost until 9 CY. That year, the combination of Oeridian divination magic and Flan lore allowed it to be located in the site where the city of Mitrik would come to be built.
The Crook was eventually brought to the Velunan city of Devarnish where, in 355 CY, it was taken as the spoils of war by invading Keoish forces. After that act of sacrilege the Crook disappeared from history until the 560s CY, when it was discovered beneath Castle Greyhawk by Murlynd, Robilar, Tenser, and Terik. Rumor has it they found it in a chamber where mindless automatons assaulted it with illogical and unreasonable riddles day in and day out; this is rumored to be one of the only ways to destroy it. The band of four continued to explore the dungeon, stumbling into the portal to the Isle of the Ape. Though they managed to escape from the demiplane, all their belongings had to be left behind, including the Crook of Rao.
Ten years later, Tenser sent a group of powerful adventures composed of Agath of Thrunch, Reynard Yargrove, Franz, Lord Torkeep, Rowena of the Silverbrow, Warnes Starcoat, and Rakehell Chert to recover the Crook from the Isle, hoping to use it to oppose the fiendish army of Iggwilv. They were evidently successful, for Iggwilv did not succeed in her plan. With the aid of the crook and an angelic host, Iggwilv’s yugoloths were banished. Their task done, the party returned the Crook to Tenser and the Circle of Eight.
The Circle, however, were not able to keep such a holy thing for long. Either they relinquished it willingly or it was taken from them, for it was next recorded in the city of Rel Astra in the hands of its animus ruler, Drax the Invulnerable.
In Late 585 CY, Drax traded the Crook back to Veluna in exchange for magic items more useful to the city of Rel Astra. In Coldeven of that year, Canon Hazen of Veluna conducted a ritual using the Crook that banished most of the demons in Iuz’s armies, as well as many other fiends all over the continent, if not the world. L’Ordre de la Croix-Rose Veritas believes that devils were exempt from this effect and only pretended to be banished so that they could work in secret. It is known that some demons managed to escape the effect due to luck or magic. It is thought the Fiend-Sage of Rel Astra avoided banishment by studying the Crook extensively beforehand.
The nature of Hazen’s ceremony is mysterious, but the Canon was attended by his entire College of Bishops and by Bigby of the Circle of Eight. In its aftermath, Hazen is said to have become enfeebled, and there are rumors that the powers of the Crook have become entirely exhausted.
In any case, the Crook of Rao remains in Mitrik, making the city the destination for devout pilgrims who view it as a source of limitless power to combat the forces of the lower planes.
At and unknow point, the Crook was used against Iggwilv. During the fight, the Witch-Queen cursed the sacred artefact. The curse infected the crook and perverted it's magical matrix. Since then, every time it's used to mass banish fiends it will end up summoning some as a collateral effect. Earlier or later, it's power will be completely inverted, and the Crook of Rao will open a planar gate, that will trough the course of 18 years, summon every fiend it ever banished. Then it will become a permanent gate to Pazunia,the first layer of the Abyss.