불리다 is the passive of 부르다 'call', and 라 marks the name: 빛의 마술사라 불리는 'known as the magician of light', 해동성국이라 불렸던 나라 'the country that was called Haedongseongguk'. It confers a title or a reputation, so it attaches to things that are famous or notorious. The fuller spoken form is -라고 불리다; dropping the 고 tightens it and sounds more written — fitting, since these lines are captions, quiz questions and narration rather than chat.