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William fitz Duncan is a modern anglicisation of either the Old French Guillaume fils de Duncan or the Middle Irish Uilleam mac Donnchada. William was a Scottish prince, a territorial magnate in northern Scotland and northern England, a fine general and the legitimate son of king Donnchad II of Scotland by Octreda.

In 1094, his father Donnchad II was killed by Mormaer Máel Petair of Mearns, supporting the claims of King Domnall III Bán. It is probable that William left Scotland with his mother in that year, to the safety of Allerdale in Cumberland. A decade or so later, he ventured to the court of his half-uncle.

Under the reign of his half-uncle Alexander I, it is highly likely that William was regarded as a viable tanaiste (i.e. "designated heir") , but Henry I of England supported David. When David succeeded, William, as the legitimate king under the rules of primogeniture was certainly bought off by David, probably being made tanaiste. William repeatedly leads the lists of witnesses appearing in Scottish royal charters in the reigns of Alexander I and David I.

A 13th century northern English source claims that William was Mormaer of Moray. As this source had no reason to make this up, it is highly likely that William was made the ruler of Moray after the defeat of King Óengus of Moray in 1130. It is feasible that this grant had something to do with the coming of age of David's son, Prince Henry. As well as being the ruler of Moray, William controlled the English lands of Allerdale, Skipton and Craven, making him one of the greatest barons of northern England.

William was a great warrior. He frequently led Scottish armies. In the campaign of 1138, he led an army of Gaels that defeated a Norman English army at the Battle of Clitheroe, raising the hopes for the success of the royal army, hopes which failed to materialize at the Battle of the Standard.

William had several marriages. His first marriage was over, presumably through death, by the year 1137, when he married Alice de Rumilly. By the latter, he had a son, also called William (William of Egremont or William the Atheling), who died in 1160. He also had a large number of bastards, probably including Wimund of the Isles. It is now presumed that Domnall mac Uilleim, and the Meic Uilleim clan that repeatedly rebelled against later Scottish kings in their quest to gain the Scottish throne, were legitimate descendants of a marriage to a daughter of Óengus of Moray.

He died in 1147, whereupon Moray fell back into the hands of David.

• Oram, Richard, David I: The King who made Scotland, (Gloucestershire, 2004)