In 1381, the Peasants' Revolt broke out in England. On 15 June of that year, King Richard II and his men met the rebels at Smithfield and killed their leader, Wat Tyler. However, what would have happened if the rebels kill the king? England would have a period of larger unrest. In the aftermath of Richard's death, two men could press their claim to the throne of England: John of Gaunt, Duke of Lancaster and Roger Mortimer. John was near Scotland, and in OTL he waited out the crisis in Scotland. In 1381, Roger was only seven years old. As such, with King Richard dead, rebellion in England became larger. John was forced to prevail upon King Robert II of Scotland for aid before things in England became even worse. In exchange for help, John was forced to sign a peace treaty with Scotland and return Scottish land that the English were occupying. John agreed to these demands and then turned his attention to England. John also sought the aid of Roger Mortimer's father, the Earl of March and Lord Lieutenant of Ireland. With the aid of the Earl of March and King of Scotland, John successfully pacified England and defeated the rebels. In the process, the Earl of March died, leaving the seven year old Roger, the technical heir to England, in John's possession. Many suspected the new King John II of having murdered Mortimer to secure his hold on the throne, laying the groundwork for the Wars of the English Succession.
Olaf II of Denmark was the son of Haakon VI of Norway and the grandson of Valdemar IV of Denmark and Magnus IV of Sweden. As such, he was in a position to become king of all three of these nations. Indeed, he became king of Denmark and Norway, but he never became king of Sweden. Olaf died in 1387 at the age of 16. In OTL, Olaf's mother Margaret united the three nations into a personal union, the Kalmar Union, but what if Olaf had lived past 1387 and established a stronger and more permanent union?
With John of Gaunt as King of England and Olaf as King of Denmark, Norway, and Sweden, history would change under the direction of The Lion Kings.