The term 'Mystical Sign' can be broken down into two crossword solutions: OMEN (4 letters) and Sign (4 letters). An OMEN is a sign or warning of something that is about to happen, often seen as a supernatural or divine indication. It is usually seen as a portent of good or bad luck. A sign is an object, quality, or event whose presence or occurrence indicates the probable presence or occurrence of something else. In this case, the sign is a mystical one, indicating something beyond the physical world.