Duke Montgomery is a human male who ruled over the city of Cordor for quite some time. He was a benevolent man at first, just and fair in his dealings. However, he began to tinker with demonic activities and eventually lost not only his compassion, but his very soul.
He, along with Earl Manfried, found the Necronomicon and attempted to use it. Duke Montgomery returned to Cordor insane and evil, while Earl Manfried became the undead he is today. When a fundraiser was held to help cure the plague that ravaged Cordor, the Duke stole the money and was subsequentily chased from town.
It was later revealed that the man who had thought to have been the duke was not him at all, the man had been replaced with a doppleganger early in his life for unknown reasons, the tendancy of the creature towards cruelty surfaced, and the rest became history.
Duke Montgomery d'Cordor, in fact, spent a quiet childhood in Comyr as a squire then purple knight. He fought as a war wizard extensivly in the variouse wars against the thayans and zhenthians.
This is incomplete and not totally accurate backstory of this npc.