Vengeful Mermaids

Mermaids in Celtic myths are always beautiful and usually friendly, even helpful to sailors and fishermen. However, when pushed, they can reveal an ugly side. In Scotland, they tell the story of the Knockdolion family who had a large house on the shore near Girvan. At night, a mermaid would come out of the water and sit on a large black rock. There she would comb her long blond hair and sing for hours. The lady of the Knockdolions felt that this serenade was annoying her baby, and ordered her servants to destroy the rock with heavy mallets. When the mermaid returned the next night and saw her favorite seat was gone, she sang:
Ye may think on your cradle--I'll think on my stane;And there'll never be an heir to Knockdolion again."
("Stane" means stone.)
Not long after, the baby's cradle was found overturned, and the baby dead beneath it. All the Knockdolion children died like this soon after they were born and the family became extinct. Celtic myths of destructive mermaids are not common but there are several.