Neo-vampiric mermaids
Revenants who resemble mermaids, a good example of this is the 'rusalka'.
The rusalka is identified in Ukrainian and Russian lore as the ghostly soul of a young woman who died by drowning or that of an infant who was stillborn or who otherwise died unbaptised. The rusalka most typically appears to people as a beautiful young woman. But she might also appear in the form of a bird or a beast, a mermaid or a young boy. A rusalka's haunt was a river, a pond, a lake, or a bay. The rusalka had a reputation for appearing to a man as a beautiful young woman, leading him with her seductive appeal into the water, and then drowning him. In some tales where a rusalka is like a mermaid, she leads the man down to her beautiful castle beneath the water and marries him. But even in these tales the rusalka has a dark side – the soul of the seduced man is doomed to end in hell.
In Brittany, there is also a legend in which a wicked princess became a siren-mermaid after the great sea-gate of her city, Ys, was opened during a severe storm at high tide and the city was totally deluged. She had been seduced and tricked by the devil, in the form of a handsome young man, into providing him the keys to the sea gate. The princess drowned with most of the other people in the city. But then she returned from the dead as a siren-mermaid.