King Arthur's cousins inform him of the love affair between Queen Guinevere and Sir Lancelot. This angers Arthur and the Queen is sentenced to death. Lancelot knowing this makes a last ditch effort to save Guinevere by taking her back to his castle.
"I would rather save your lives than rule all that Alexander possessed while he while he lived upon this earth!"(page 321)
The siege on Lancelot's castle did not work, but to everyone's surprise Lancelot returns Guinevere to Arthur. Sir Gawain convinces Arthur to follow Lancelot across the ocean. While they were across seas Mordred had seized the throne. Sir Gawain takes a fatal blow, and with his final breaths he writes a letter to Lancelot calling on him to assist Arthur.
A war between Arthur and Mordred takes place as a fight for the throne. Arthur kills Mordred but not before Mordred strikes Arthur with a fatal blow. Arthur is taken to a church to heal but ends up dying from his wounds a few days later. After Arthur's death Guinevere becomes a nun and Lancelot a priest. Each trying to repent for their sins.
I think what makes Arthur such a memorable king is because of his good character and equality mindset that was uncommon for his time. Along with that he also had an unusual upbringing and how he becomes king is just ad unusual. All these things together are the reason we see so many movies featuring one of history's greatest kings. King Arthur.