Kenneth I (a.k.a.Cináed mac Ailpín, Kenneth Mac Alpin, and Kenneth the Hardy) lived from 810 to 858 and was arguably the first King of the Kingdom of Scotland, which he ruled from 843 to 858. At the time he was referred to as King of the Picts. The wider picture in Scotland at the time is set out in our Historical Timeline.
He was son of King Alpin II of Dalriada and succeeded his father to the crown of Dalriada in 839. This effectively made him King of the Scots, whose territory roughly covered modern-day Argyll. Meanwhile, also in 839, the Picts, who until then had controlled all of Scotland north of the Forth and Clyde except for Argyll, suffered a heavy defeat at the hands of the Vikings. Most of the Pictish nobility was wiped out in the defeat, including King Bridei VI.
Kenneth Mac Alpin had a claim to the Pictish crown through his mother. But his claim was disputed by surviving members of the seven royal houses of the Picts, and Drust X succeeded to the Pictish Crown. Kenneth defeated the Picts in battle in 841: and squeezed between the Scots on one side and the rampaging Vikings on the other, the Picts agreed to a meeting with Mac Alpin at Scone, attended by all claimants to the Pictish Crown.
The alcohol flowed freely at the meeting. Then, in what has since been referred to as Mac Alpin's treason, Drust and the Pictish nobles were all killed by the Scots: allegedly (and improbably) by having their booby-trapped benches collapsed so Kenneth's rivals plunged into pits in the floor and impaled themselves on spikes set there for the purpose.
Suddenly there was only one claimant for the Pictish Crown, and Kenneth was crowned King of the Picts and the Scots in 843. He was the first King of the House of Alpin, the dynasty named after his father. Kenneth made his capital at Forteviot, a small village 5 miles south west of today's Perth. He also moved the religious focus of his kingdom from Iona to Dunkeld, and had St Columba's remains moved there in 849, perhaps for safe keeping from the continuing Vikings raids.
Mac Alpin continued to fight against Picts who challenged his right to hold their crown, but by 855 his grip on those parts of modern Scotland north of the Clyde and Forth not under the control of the Vikings was relatively secure. He had also created some sort of stability in his relations with the Britons and the Angles who held the lands to the south.
Over time his combined Kingdom of the Picts and Scots came to be referred to as Alba: later know by medieval scholars (rather confusingly) as Albania.