Duff was the eldest son of Maol Chalvim I, born in 912. When Maol Chalvim died, he placed his widow (and Duff's stepmother), Queen Katharine, in the charge of Duff and his younger brother Kenneth. The rival lord Indulf took advantage of this to take Katharine prisoner while Duff and Kenneth were taking Maol Chalvim's body to Iona for burial, and held her hostage to force them to recognize his usurpation of the Scottish throne.
In 962, Katharine and her son Prince Malcolm, with help from Robbie, finally freed themselves from Indulf's captivity, and thereby forced him to abdicate in favor of Duff. Duff ruled Scotland well for five years, until Indulf's son Culen returned in 967 and fought against him at the Battle of Gaine. On the first day of the battle, Duff was badly wounded, so much so that he could no longer walk. So on the second day of the battle, he was taken to the field in a horse-litter, so that his presence could at least inspire his fighting-men. Unfortunately, he was treacherously murdered by one of his bearers (who was presumably suborned by Culen). With his death, Culen won the Battle of Gaine, and Duff's family was forced to flee to northern England.
Duff was the father of Kenneth III of Scotland (997-1005).
Real World History
Duff was an actual King of Scotland, although little is known about him. In real history, he was murdered by the governor of Forres Castle in 967; this murder was used by William Shakespeare as the inspiration for his account as to how Macbeth and Lady Macbeth murdered Duncan in his play Macbeth. The account in the Gargoyles Universe of Duff going into his final battle in a litter evokes the tale found in both Geoffrey of Monmouth and Sir Thomas Malory of how Uther Pendragon, too ill to fight on foot or ride on a horse, similarly fought the Saxon invaders of Britain at Verulamium (now St. Albans) in a horse litter - though Uther, unlike Duff, was victorious.