Who was the first King of the Scots?

The Gaelic kingdom of Dál Riada encompassed much of the southwestern seaboard of what is now Scotland. In 841, King Coinneach mac Ailpein, more commonly Anglicized as Kenneth MacAlpin, was crowned  King of Dál Riada, inheriting it from his father, who was killed in battle with Wrad, King of Pictavia. Kenneth's mother was a Pictish princess.

To the east was the Kingdom of the Pictavia, the largest and most fertile kingdom in early medieval Scotia. Wrad soon died and was succeeded by his son, King Bred.

King Kenneth was both an ambitious man and one under serious pressure. The Vikings were attacking Dál Riada in the west. They conquered its coastlands. Either Kenneth took action, or his kingdom might cease to exist.

The Picts had driven off both the Romans and expelled the Angles. However, they also were being slaughtered by a Viking invasion. This was a fact King Kenneth was happy to take advantage of, especially since, in Gaelic Dál Riada, inheritance could be through the female line. So Kenneth had a claim to the throne of Pictavia.

As the author of the Chronicle of Huntington put it:

Kynadius, passing into their remaining territories, turned his arms against them and, having slain many, compelled them to take flight, and was the first king of the Scots who acquired the monarchy of the whole of Alban, and ruled in it over the Scots.

The Pictish king, King Bred, fell in battle against King Kenneth and his men. From that bloody battlefield, the Scots and Picts emerged one nation. In fact, myth to the contrary, the Picts were not wiped out. The Gaels migrated in large numbers into the more fertile and accessible Pictavia. King Kenneth moved his government to Scone in the east of the kingdom, brought the kingdom's most important relic, the Stone of Destiny, from Iona, and ruled as the two kingdoms peacefully merged.

But peace was not to be. The Vikings conquered the entire coast of Dál Riada. Without the strength to expel the Vikings, he turned his armies south, invading the part of (then) Northumbria north of the River Tweed. Time after time, he invaded, burning towns as he went. By his death, he still had not managed to take what would eventually be southeastern Scotland, but he laid the groundwork for it.

In 858, King Kenneth I died peacefully in his bed from a tumor. Because succession was by tanistry (inheritance by second-in-command), his brother King Donald I inherited the throne of Alba, forming the Alpínid dynasty, which ruled Scotland for two hundred years.