The Mermaid Demons are minor antagonists in Doraemon: Nobita's Great Adventure into the Underworld. They're the servants of Demaon and were tasked to lure the gang to their island with their voice and had their ally, the Whale Demon, to try to devour them. They almost succeed in their plan until Gian unleashed his own destructive singing voice, scaring the Mermaid Demons and forcing them to flee as well as disabling the Whale Demon. With them gone, the gang were able to cross the ocean to the mainland.