Aldfrith was King of the Northumbrians from 685 to 696. He was the half-brother and successor of Ecgfrith, and the son of King Oswiu by his first marriage.
Aldfrith was born and brought up in Dal Riata, where his father had been exiled by King Eadwine. His mother was Fín, daughter of Colmán Rímid of the Uí Néill. Aldfrith grew up amidst the Irish language and culture, and continued to maintain strong ties with Ireland and Dal Riata after his accession to the throne.
After Ecgfrith was killed in battle with the Picts in 685, Aldfrith was able to use Irish support to claim the throne of Northumbria himself. He quickly set about continuing Ecgfrith's policy of trying to maintain Northumbrian dominance in northern Albion, and from 687 to 690 waged war against the Britons of Strathclyde. His warbands swelled by Angle and Saxon refugees from southern Albion, he was able to crush Strathclyde and forced its people to swear allegiance to him as king.
During the early 690s Aldfrith came into conflict with Powys and Gwynedd, which lay on his southern border, culminating in an outright invasion of Northumbria in 696. In April of that year Aldfrith was defeated and captured at the Battle of Loidis, and without his leadership all southern Northumbria was quickly lost to the Britons. Aldfrith lived until 705 has a hostage in Gwynedd, while the remainder of his kingdom was usurped by Eadwulf the Bernician.