Constantine II (a.k.a. Causantín mac Áeda) lived from 874 to 952 and was King of Alba from 900 to 943. The wider picture in Scotland at the time is set out in our Historical Timeline.
He is perhaps best known for finally driving the Vikings out of mainland Scotland following the Battle of Scone in 904: though not before the Vikings had destroyed much of Alba. He then allied with the Northumbrians to take on the Vikings who had settled in Northumberland, defeating them at the second Battle of Corbridge, in 918. The result was, for the first time, that Alba had a fairly stable southern border.
Constantine reformed Alba in a number of ways. Under his reign the church became more Gaelic in its approach, and he introduced a system of Mormaer: earls or sub-kings responsible for looking after parts of the kingdom.
Building on improved relations with the Vikings, Constantine's daughter married Olaf III Guthfrithson, the Viking King in Dublin, in 937. The immediate result was an unhappy one. A joint army comprising Constantine's Scots and Olaf III Guthfrithson's Vikings was defeated at the Battle of Brunanburh by King Athelstan of England in 937. Amongst the dead was one of Constantine's sons, Cellach. This battle, whose location might have been anywhere from south-west Scotland to Humberside, was one of the most significant in the history of these islands as it effectively confirmed the status and approximate borders of "England". Following the battle the Scots abandoned Lothian, including Edinburgh, to Northumbrian rule.
In 943 Constantine II abdicated, leaving the way clear for Malcolm I to succeed him. He retired to the Culdee Monastery at St Andrews in Fife, where he was probably buried after his death in 952. Despite his early abdication, Constantine's was the second longest reign of any Scottish monarch.