The King of Orkney. According to Geoffrey of Monmouth, the famous historian of Celtic legends, Lot was the brother of Urien, king of Moray, and Auguselus (Angel), king of Albany. He led the British forces against the Saxons. According to the historian, King Arthur gave Lot the lands of Lothian and Orkney as a reward for his loyalty.
He was the father of the Orkney Faction: Gawaine, Gaheris, Gareth, and Agravaine, knights who, although they served in the Round Table, resented Arthur for his father's rape of their grandmother, Lady Ygraine. Family and devotion to their lands were principal virtues within the Orkney Faction, and one of the driving forces behind King Lot's actions.
The rivalry between Lot and Arthur is a definitive part of the Arthurian legend, and one that has further reaching implications than a mere interfamilial feud. Lot refused to accept Arthur as King despite his prophesized pulling out of The Sword in the Stone, and formed an alliance with eleven other kings to stop him. Beaten at the Battle of Bedegraine by Arthur's disregard for the "knightly code of war", Lot fades into the background, and eventually becomes neutral towards Arthur. However, war is no paltry matter, and Queen Morgause and Sons continue with their plans for Arthur's demise (although Morgause is the most insistent on a cruel and unusual punishment). Mordred is born (the illegitimate love child of Arthur and Morgause) and although T.H.White's version of the events does not give any indication of Lot's anger, Mary Stewart's "The Last Enchantment" does, saying that it was Lot and not Arthur who ordered Mordred's execution. Again, Lot's high regard for family ties comes into play here in this decision.
Lot was killed at the hands of King Pellinore, the old, absent minded king without a kingdom, constantly after "The Questing Beast." This started a string of vengeful killings that deepens the tragedy of Arthur, and seals in the bitter fate of the Pendragons. Lot's overprotectiveness of family and land killed him in the end.