The video above, 5 Game of Thrones Plotlines Ripped Right Out of History, contains spoilers. I watched the video and had to look up the Black Dinner. It was basically a set-up: a ten-year-old Scottish king invites a teenage nobleman to a royal banquet and everyone dies. Well, the three out-of-town guests died. When you have a child as king, there are always those who are scheming to be the power behind the throne. In Scotland in 1440, Sir Alexander Livingston, the king’s guardian, and Lord Chancellor Sir William Crichton held that power. They felt under threat by 16-year-old Earl William Douglas. Douglas led a powerful clan who could consolidate their political power in the future. The title of the young earl was also coveted by his great-uncle James, who conspired with Crichton and Livingston to get rid of William and his brother. From Robert Sewell's Genealogy Site:
It does not appear to have been difficult for Sir William Crichton to lure the young earl from his castle, and to convince him to present himself at the court of the boy king, James II in Edinburgh Castle for a celebratory dinner of reconciliation. Thus, the 6th Earl of Douglas, his brother David, and his advisor Sir Malcolm Fleming of Cumbernauld arrived at Edinburgh Castle on November 24, 1440.
According to legend, a banquet was held in the Great Hall of Edinburgh Castle, and the young James II was charmed with the company of the Douglases. At the end of the feast, the head of a black bull was brought into the hall. Under Scottish custom, this formality presaged the death of the principal guest(s) at a dinner. James II is alleged to have pleaded for the lives of his new friends to be spared, but they were said to have been beheaded in front of the ten year old king.
However, Mr. E.B. Livingston suggests a more likely scenario on pages 43 and 44 of The Livingstons of Callendar, Edinburgh University Press, 1920:
"But what we do know for certain is that on the arrival of the Earl of Douglas at the castle, he was at once arrrested, together with his only brother David, and his friend and consellor Sir Malcolm Fleming of Cumbernauld, who had accompanied him; that the three of them were hastily tried for high treason, found guilty, and proptly beheaded on the Castle Hill. The earl and his brother were executed on 24 November, 1440, and Sir Malcolm Fleming four days later. The later execution must have been carried out contrary to the wishes of Livingston, hence probably the four days' delay. For about three years later, on 16 August 1443, Sir Alexander Livingston, in the presence of Robert Fleming and four bishops, solemnly purged himself upon oath of having given any counsel, assistance, or consent to the slaughter of Sir Malcolm Fleming.
Read a more detailed version here. The assassination is well-documented, although many historians believe the banquet and the black bull head are legendary.