Sentence:  In medieval times, the image of the double fish tailed siren was called a <WSD>nixie</WSD> or evil water spirit. Yet, the <WSD>nixies</WSD> were more likely daughters of the primordial goddess named Nyx or mother night, who hovered over ancient waters, rather than evil spirits. 
Meaning of the word marked with <WSD> tags:  A female nix, a water-spirit.