MacAlpin's Treason, as it is now called, was a murder of the nobles of the Pictish realm of ancient Scotland. Kenneth mac Alpin's Pictish mother was descended from the royal house of Fortrenn, and his great-grand uncle, Alpin Mac Eachaidh, had actually reigned as King of Picts until deposed by Oengus I. It is thus that Kenneth Mac Alpin was one of several nobles with a claim to the crown of Picts and Scots.
The sources for facts of how Kenneth Mac Alpin, the avenging son of the slain Alpin, became King of Picts and Scots are few and suspect. Two such sources, The Prophecy of St. Berchan, and De Instructione Principus note that in 841 Mac Alpin attacked the remnants of the Pictish army and defeated them.
Mac Alpin then invites the Pictish king, Drust IX, and the remaining Pictish nobles to Scone to perhaps settle the issue of Dalriada's freedom or MacAlpin's claim to the Dalriadic crown. Faced with a recently victorious MacAlpin in the south, and a devastated army in the north, Drust, as well as all claimants to the Pictish throne from the seven royal houses attend this meeting at Scone. Legend has it that the Scots came secretly armed to Scone, where Drust and the Pictish nobles were killed.
It is Giraldus Cambrensis in De Instructione Principus who recounts how a great banquet was held at Scone, and the Pictish King and his nobles were plied with drinks and became quite drunk. Once the Picts were drunk, the Scots allegedly pulled bolts from the benches, trapping the Picts in concealed earthen hollows under the benches; additionally, the traps were set with sharp blades, such that the falling Picts impaled themselves (The Prophecy of St. Berchan tells that "...[Mac Alpin] plunged them in the pitted earth, sown with deadly blades...") . Trapped and unable to defend themselves, the surviving Picts were then murdered from above and their bodies, clothes and ornaments "plundered."
