The problem of Strathclyde became increasingly linked with another, potentially more dangerous external threat - that of Northumbria, which was taken by the Danes in 866. The rise of a Viking based in York was the steady accompaniment against which the story of the mac Alpin dynasty had to be played out. A clue to the seriousness with which tenth-century mac Alpin kings took the new Northumbrian threat lies, again, in the names they chose for their sons - who were not their immediate successors but kings in waiting with the rígdamna. Kenneth mac Alpin, like kings of Picts before him, had deliberately chosen alternate Roman and Gaelic names for his first two sons: the cult of Pictish kingship and the cultivation of his own Gaelic past stood side by side in the figures of Constantine and Aéd. Yet by the early tenth century, Constantine II chose a Scandinavian name for his son Indulf, who eventually succeeded in 954: Indulf in his turn gave a Scandinavian name to one of his sons, Olaf.

The Scandinavian threat was real. Eóganán, song of Óengus, had been slaughtered along with 'others almost without number' by Norsemen in 839, precipitating a crisis in the succession. Constantine I had died along with a 'great multitude' in battle against the Danes in 877 when, it was said, 'the earth burst open under the men of Alba.' Defeat at the hands of hostile invaders again provoked an internal crisis: Constantine was succeeded by his brother Aéd who lasted less than a year until he was, according to the Annals of Ulster, ousted by him own people. The death of the last of Kenneth mac Alpin's sons left the succession in a state of confusion, which was seemingly patched up only by the expedient of a joint kingship, shared between one of the conspirators, Aéd's first cousin Giric, and one of Kenneth's grandson's Eochaid, a son of the King of Britons. The Strathclyde dynasty had itself good reason to be wary of Scandinavian aggression, for Al Cloth, its capital on Dumbarton rock, had fallen after a four -month siege in 870 and large numbers of captives had been ferried to Dublin in a huge fleet of 200 longships. The early years of the long reign of Constantine II were also fraught with danger: in 903 Norsemen plundered Dunkeld ' and the whole of Alba' but were trapped in Strathearn in the following year by the 'men of Fortriu' who rallied behind the 'Battle Victory,' of crozier of Columba. In 918 the same emblem led the Scots into battle at Corbridge against a potentially more serious enemy, a large force of Danes led by Ragnall, King of York.