William fitz Duncan (a modern anglicisation of the Old French Guillaume fils de Duncan and the Middle Irish Uilleam mac Donnchada) was a Scottish prince, the son of King Duncan II of Scotland by his wife Ethelreda of Dunbar. He was a territorial magnate in northern Scotland and northern England and a military leader.
In 1094, his father King Duncan II was killed by Mormaer Máel Petair of Mearns, supporting the claims of King Domnall (Donald) III Bán. It is probable that his mother Ethelreda took the infant William and fled Scotland to the safety of Allerdale in Cumberland where her brother Waltheof of Allerdale was lord. William, an only child, grew up there among his cousins. 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 tánaiste (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 tánaiste. 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 deceive, 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, as they coincided closely with each other, that this granting of vast, important territory was in exchange for being passed over by David in the line of succession with the coming of age of David's son, Prince Henry to whom David wanted to secure as his successor, and may also be a factor in his marriage to a daughter of Óengus, whose lands he was receiving. Granting William the Mormaerdom of Moray may have had the dual purpose of compensating a relative for excluding him from succession as well as placing one of David's most loyal and worthy agents over a fragile and recently acquired territory, which he was very keen on keeping. In addition to being the ruler of Moray, William also controlled the English lands of Allerdale, Skipton and Craven, making him one of the greatest barons of northern England.
William was an excellent warrior. His uncle, King David, frequently tasked William with leading his armies in battle. 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. Accordingly, perhaps disappointingly for David for his plans of colonizing and better integrating Moray into his kingdom, William fitz was usually absent from the province because of campaigns, as well as his duties over his other territories.