Duncan was a grandson of Maol Chalvim II and cousin of Macbeth. By 1020, he was already the heir apparent to the Scottish throne, but feared that Findlaech would make Macbeth King of Scotland instead. To prevent this, he employed Gillecomgain the Hunter to assassinate Findlaech, and bestowed upon him the High Stewardship of Moray as a reward ("City of Stone" Part One).
In 1032, after Duncan's oldest son Canmore was born, Duncan decided to ensure all the more that Macbeth would not be able to steal the throne from him by commanding Gillecomgain to murder Macbeth. When Gillecomgain refused, Duncan retaliated by informing Macbeth that Gillecomgain was the Hunter who had murdered Findlaech, resulting in Macbeth slaying Gillecomgain for him. Duncan afterwards succeeded Gillecomgain to the role of the Hunter ("City of Stone" Part Two).
By 1040, Duncan had become King of Scotland at last. While out for a stroll on the heath with Macbeth and their sons, Canmore and Luach, he first learned of Macbeth's alliance with Demona when he came upon her and her clan in their stone sleep), and then encountered the Weird Sisters who prophesied that Macbeth would someday be King of Scotland. His suspicions towards Macbeth aroused once more, Duncan promptly determined to destroy him for good, and also decided to destroy Demona's Clan in case Macbeth allied with it against him. His attack upon both Clan Moray and the gargoyles forced Macbeth and Demona to become allies, and they joined forces to defeat him in battle. In that battle, Duncan was finally slain when he brought his sword down upon a magical ball that Seline had given Macbeth, which responded by destroying him in a blast of fire ("City of Stone" Part Three).