Constantine I (a.k.a. Causantín mac Cináeda) lived from 836 to 877 and was King of the Picts and Scots from 863 to 877. The wider picture in Scotland at the time is set out in our Historical Timeline.
Constantine spent most of his reign fighting to consolidate and expand upon the Kingdom of Scotland created when Kenneth I had brought together the crowns of the Scots and the Picts to form the beginnings of what became known as Alba. To the south he battled against the King of Strathclyde. This culminated when he arranged the death of the King of Strathclyde in 872. He replaced him with his own brother in law, Rhun: effectively making Strathclyde a subordinate kingdom to Alba.
On other fronts Constantine faced constant pressure from the Vikings. In quick succession in the mid 860s Constantine defeated Norse forces led by Olaf the White and by Thorsten the Red.
But in 877, Vikings from Dublin and Yorkshire landed in Fife and established a base from which to raid much of the eastern side of Scotland. King Constantine I of Scotland died in battle against the Norse at Inverdovat, on the outskirts of today's Newport on Tay in North East Fife. He was buried in the graveyard at Saint Oran's Chapel on the Isle of Iona. Constantine was briefly succeeded by his brother, King Aedh.