What plot archetype is used in the legend of King Arthur?

well Merlin can be seen as a mentor, and a companion, to both Uther Pendragon and Arthur.

Arthur can be seen as a mentor for the people. He is a noble king that uses his power for the good of the people.

Beaumains/ Sir Gareth can be seen as the Initiate Hero, or Hero Incognito, because he rises from nothing into a very noble knight that rescues a damsel in distress, after honorably fighting the Red, Green, and Black knights, as well as Sir Persuant in the Knight of the Kitchen.

Sir Mordred is the malevolent nature, nightmare, beast, fury, or monster.