The Singing Ice (5E)

From the publishers blurb: The party enters a castle that at first seems to be abandoned. But something is not right. It is empty and cold but clean and cared for. The princess has been turned into a beast. The spell will not be broken until a special crystal is placed in the highest tower. If that is done, it is proof that the princess learned to ask for help. The crystal is found on the other side of a portal, guarded by a dragon that should not be awoken.