You can look up King Arthur: The Role-Playing Wargame. Yeah, it was published by Paradox, but whatev.

Think CA games, but with more RPG elements like heroes that can go on quests on the campaign map etc. I don't really recall the story, but initially it starts relatively "medieval", but then the fey beings start invading. 2nd game also has an expansion about Romans in Britannia.

The concept of the game was nice, but it definitely had issues.