The term 'Sign of Things to Come' can be broken down into four crossword solutions: PORTENT (7 letters), OMEN (4 letters), HARBINGER (9 letters), and COLON (5 letters). A PORTENT is a sign or warning of something important or calamitous that is about to happen. An OMEN is a sign or warning of something that will happen in the future, often seen as a supernatural sign. A HARBINGER is a person or thing that foreshadows or foretells what is to come. Finally, a COLON is a punctuation mark used to introduce a list or a quotation. All of these words refer to signs that indicate something is about to happen in the future.