Well, there are different things the audience can be surprised by. They can know that something is coming, and still be surprised by it when it happens, if what they are expecting is smaller or different than what actually comes. This proposed greedy Lorien-Maia turning into MEGA-DRAGON is pretty surprising, even if we know Lorien-Maia exists, and even if we know that he is being... modified by Morgoth. It's still a startling left turn. Especially if we can pair it with an aggressive jump-scare when he is first revealed.