The quest you need to complete in order to earn King Artorious is the Letters of Light quest, which opens up Khrysalis. Once you've finished Avalon, next is all of Azteca and then, finally, you reach Khrysalis and get your Artorious spell.
(Not really relevant but level 90 at the end of Avalon? Thats pretty amazing dude)
You don't get King Artorious at the end of Avalon, instead you get it at the beginning of Khrysalis. This is when youre gathering members for the coucil of light. You ask King Artorious but he doesnt want to leave while Avalon is still weak. But he gives you a card to call him whenever you need him.
They say "Level 85 minimum" because the level limit for that world is 85.