Miyazaki says that the pair had "many free and creative conversations... which Mr Martin later used as a base to write the overarching mythos for the game world itself." So by this he's responsible for the world's founding lore. "This mythos proved to be full of interesting characters and drama along with a plethora of mystical and mysterious elements as well," added Miyazaki. "It was a wonderful source of stimulus for me and the development staff. Elden Ring’s world was constructed using this mythos and stimulus as a base."

From an interview.