Merilla is a mermaid queen from 1918 movie Queen of the Sea, portrayed by actress Annette Kellerman.
Merilla finds a book prophesying her getting a mortal body and immortal soul after saving four human lives. Boreas, master of the storms, wreck many ships, and sends sirens to drag the men to certain death. Merilla saves four lives, one of them is a prince, and they fall in love with each other. Ariela tells them not be selfish, and Merilla lets her prince go, since he was betrothed to a princess. The princess however gets kidnapped by Boreas, and even though Merilla is now human and mortal she decides to save the princess, even if it means losing the prince she loves. Merilla tells the prince where his fiancee is, and he arrives with his knights to save her. The princess then confesses her love for prince's courtier, and releases the prince from his vows. Merilla and the prince now can get married.