September 4, 1241 - March 19, 1286: Age 44
Alaxandair mac Alaxandair was the only son of King Alexander II (by his wife, and thus the only one that counted). He became king at the age of 8 and, typically, his early years as king were dominated by power struggles between factions struggling for control of the regency, which included at one point Alexander being kidnapped by one of the factions.
When he turned 21 he took charge decisively, and proved to have a good understanding of power. He married the daughter of the King of England but managed to rebuff Henry III's attempts to extract homage from him. He played the King of Norway for a fool, laying claim to the Western Isles and then stringing out negotiations until the Norwegian king lost patience and attempted an invasion during the storm season...in sea-to-land battles, storms fight on the side of the defender. The fleet was defeated and the king died on the way home. Later Alexander married his daughter to the new King of Norway.
Alexander was unlucky in one important matter, though: all three of his children predeceased him, leaving him with one three-year-old granddaughter as sole heir. In the 1200s that meant, to many, no heir at all. He was on his way to visit his new, pregnant wife when he fell from his horse in the dark while riding to visit his new queen. He had become separated from his companions, so nobody actually saw it happen, but he was found dead on the shore below a steep rocky embankment the following morning.
His wife later delivered a stillborn son, and a group of Earls took over governance of the realm until a husband could be found for Alexander's granddaughter, who lived in Norway. Four years later, she was on her way back to Scotland when she herself died. By this time the kingdom was seething with rebellions by opportunistic nobles, and Edward I of England turned a covetous eye northward. His designs on Scotland resulted in a 300-year struggle in which the Scots, though overmatched, fought back ferociously. It ended in 1603 when King James VI of Scotland ascended the English throne as heir to the childless Elizabeth.