King Duncan I (or Donnechad mac Crinan in Gaelic) was born in 1001. His mother was Princess Bethoc (Beatrix). His father was Crinan, the Lay Abbot of Dunkeld. He had a brother named Maldred. In 1018 Owen the Bald died leeving the throne of Strathclyde open for Duncan. He was then king of Strathclyde at age 17..
In 1030 at age 29 he married Sibyl of northumbria. In 1034 at age 33 he became king of Scotland. He was the first king of a united Scotland. He was also Scotland's first priest king because he was son of and hereditary heir to his father Crinan, the Lay Abbot of Dunkeld. The idea of a priest monarch was from then on embedded in Scottish culture.
Soon after his succession to the throne There was a series of riots that even the captain of his army, Banqo, could not contain. A military council was created and Macbeth managed to gain control of King Duncan's army. He managed to subdue the revolt and by doing that he managed to get more popular with the king. In 1040 he invaded Northumbria to the south and Caithness at the same time. He lost four battles in a row ending with his death at Burghead. He was killed by his own men led by Macbeth after a battle. They were really unhappy with him.