Näkki

A Näkki is a mythical creature said to be an evil spirit or guardian of a river, spring, bridge, pond or well. Näkki often bullies people, and can even take someone away. The appearance of this also varies in different stories and pictures. Quite often it is a shapeless lump covered with aquatic plants that may resemble a lizard or a frog. Näkki might be a handsome young man or a beautiful maiden who seduces an unsuspecting passerby in order to exploit him in the depths.
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“You say hello to Näkki and ask how she is doing. She says she is fine, and both of you sit in silence for a minute before just sort of...going to do...something else. Ahem.”

“You move towards Näkki to greet her affectionately, but she head-butted you in greeting too hard. Ouch, that smarts a bit.”