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Causantín mac Cináeda (Modern Gaelic: Còiseam mac Choinnich; died 877) was a king of the Picts. He is often known as Constantine I in reference to his place in modern lists of Scottish monarchs, but contemporary sources described Causantín only as a Pictish king. A son of Cináed mac Ailpín ("Kenneth MacAlpin"), he succeeded his uncle Domnall mac Ailpín as Pictish king following the latter's death on 13 April 862. It is likely that Causantín's reign witnessed increased activity by Vikings, based in Ireland, Northumbria and northern Britain. He died fighting one such invasion.

Writing a century before Causantín was born, Bede recorded five languages in Britain. Latin, the common language of the church; Old English, the language of the Angles and Saxons; Irish, spoken on the western coasts of Britain and in Ireland; Brythonic, ancestor of the Welsh language, spoken in large parts of western Britain; and Pictish, spoken in northern Britain. By the ninth century a sixth language, Old Norse, had arrived with the Vikings.

Viking activity in northern Britain appears to have reached a peak during Causantín's reign. Viking armies were led by a small group of men who may have been kinsmen. Among those noted by the Irish annals, the Chronicle of the Kings of Alba and the Anglo Saxon Chronicle are Ívarr—Ímar in Irish sources—who was active from East Anglia to Ireland, Halfdán—Albdann in Irish, Healfdene in Old English— and Amlaíb or Óláfr. As well as these leaders, various others related to them appear in the surviving record. Viking activity in Britain increased in 865 when the Great Heathen Army, probably a part of the forces which had been active in Francia, landed in East Anglia. The following year, having obtained tribute from the East Anglian King Edmund, the Great Army moved north, seizing York, chief city of the Northumbrians. The Great Army defeated an attack on York by the two rivals for the Northumbrian throne, Osberht and Ælla, who had put aside their differences in the face of a common enemy. Both would be kings were killed in the failed assault, probably on 21 March 867. Following this, the leaders of the Great Army are said to have installed one Ecgberht as king of the Northumbrians. Their next target was Mercia where King Burgred, aided by his brother in law King Æthelred of Wessex, drove them off. While the kingdoms of East Anglia, Mercia and Northumbria were under attack, other Viking armies were active in the far north. Amlaíb and Auisle (Ásl or Auðgísl), said to be his brother, brought an army to Fortriu and obtained tribute and hostages in 866. Historians disagree as to whether the army returned to Ireland in 866, 867 or even in 869. Late sources of uncertain reliability state that Auisle was killed by Amlaíb in 867 in a dispute over Amlaíb's wife, the daughter of Cináed. It is unclear whether, if accurate, this woman should be identified as a daughter of Cináed mac Ailpín, and thus Causantín's sister, or as a daughter of Cináed mac Conaing, king of Brega. While Amlaíb and Auisle were in north Britain, the Annals of Ulster record that Áed Findliath, High King of Ireland, took advantage of their absence to destroy the longphorts along the northern coasts of Ireland. Áed Findliath was married to Causantín's sister Máel Muire. She later married Áed's successor Flann Sinna. Her death is recorded in 913. In 870, Amlaíb and Ívarr attacked Dumbarton Rock, where the River Leven meets the River Clyde, the chief place of the kingdom of Alt Clut, south western neighbour of Pictland. The siege lasted four months before the fortress fell to the Vikings who returned to Ireland with many prisoners, "Angles, Britons and Picts", in 871. Archaeological evidence suggests that Dumbarton Rock was largely abandoned and that Govan replaced it as the chief place of the kingdom of Strathclyde, as Alt Clut was later known. King Artgal of Alt Clut did not long survive these events, being killed "at the instigation" of Causantín son of Cináed two years later. Artgal's son and successor Run was married to a sister of Causantín. Amlaíb disappears from Irish annals after his return to Ireland in 871. According to the Chronicle of the Kings of Alba he was killed by Causantín either in 871 or 872 when he returned to Pictland to collect further tribute. His ally Ívarr died in 873.

In 875, the Chronicle and the Annals of Ulster again report a Viking army in Pictland; the Annals of Ulster say that "a great slaughter of the Picts resulted". No name is given to the battle in which the slaughter occurred, yet the Chronicle notes a battle fought between Danes and Scots near Dollar but notes a subsequent "annihilation" at Atholl. In 877, shortly after building a new church for the Culdees at St Andrews, Causantín was captured and executed (or perhaps killed in battle) after defending against Viking raiders. Although there is agreement on the time and general manner of his death, it is not clear where this happened. Some believe he was beheaded on a Fife beach, following a battle at Fife Ness, near Crail. William Forbes Skene reads the Chronicle as placing Causantín's death at Inverdovat (by Newport on Tay), which appears to match the Prophecy of Berchán. The account in the Chronicle of Melrose names the place as the "Black Cave," and John of Fordun calls it the "Black Den". Causantín was buried on Iona.

Causantín's son Domnall and his descendants represented the main line of the kings of Alba and later Scotland.