The term 'Mysterious Sign' can be solved by the crossword solution 'OMEN', which is a four-letter word. An OMEN is an event or occurrence that is believed to portend a good or bad future event. It is often seen as a sign of what is to come, and is usually interpreted as a warning or a prophecy. OMEN can also refer to any kind of supernatural sign or event that is seen as a warning or indication of a future event.