Well, I know of two widely told versions of the death of legendary King Arthur. One theory is that Lancelot — once Arthur's most trusted knight — and Queen Guinevere betrayed Arthur, which prompted Arthur to go to war against Lancelot. Before going away for battle, Arthur left Mordred (his nephew) temporarily in charge of Camelot. But power-thirsty Mordred soon wanted the kingdom for himself, which resulted in a swordfight between Mordred and Arthur that ended in the deaths of both of them.
How does King Arthur die?
The other, more mythical account is that Arthur never died at all. Instead, after the swordfight he was taken to the magical island of Avalon. On Avalon, Arthur recovered from his wounds and still waits to return to England at some dire, future time when his countrymen need him the most. This legend is why Arthur is often referred to as "the once and future king."