The Blade of Veysgo Hallan is an artifact of great power, a dagger that can kill a wizard by utterly destroying their essence.
In days of yore, Veysco Hallan was an infamous hunter and killer of wizards, who eventually discovered the secret of making those deaths permanent.1
It was discovered by Darby Barleyfoot at some point in his travels, and hidden behind his portrait in The Vermilion Minotaur.2
Riddle and concealment
A mysterious old man who Arnie and Usidore encounter in the basement of the Vermilion Minotaur gives them the following riddle:
It services good king Irving Clark
Rendered where rasher foes embark
Behind are words no need to go
But find the final undertow3
Arnie identifies Irving & Clark as the intersection in Chicago where he fell into the portal to Foon, and listeners on Earth find additional clues that point Arnie, Chunt, and Usidore to the painting of Darby (though it does raise the question of who could plant clues on two separate worlds). Usidore plans to use the blade to kill the Dark Lord at his birthday party, but is persuaded to wait when the Dark Lord proposes a truce to defeat the Void.
Having obtained Obsidian Doom, a second weapon cable of destroying wizards, Arnie later trades the Blade of Veysco Hallan to the conviction artist Rapunsale for a ladder out of her tower.4