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I was thinking that the mermaids would try to drown others, particularly women in order to make their numbers grow. I was thinking that the mermaids would be drowned women who were possessed by an evil spirit, does that sound fine to you?

Maybe with fairies they want their numbers to grow so they abduct children and raise them as their own. They could be the equivalent of certain stories that parents use to teach their kids to be good. Maybe there's a few festivals per year where the fairies are offered gifts in exchange for leaving children alone?