The term 'Nautilus Leader' is a reference to the character Captain Nemo from Jules Verne's classic novel, 20, 000 Leagues Under the Sea. The crossword solution for this term is 'OMEN, ' which is a sign or warning of something to come. In the novel, Captain Nemo is a mysterious figure who commands the Nautilus submarine and leads his crew on a journey of exploration and discovery. He is a leader who is both feared and respected by his crew, and his presence is often seen as an OMEN of what is to come.