Nøkks (also referred to as Necks, Nicors, Nixies) are Norse water spirits. They are made completely out of water and use violins to make their victims cry until they literally die of broken hearts.
While Alderman was in possession of a cursed ring, he came under the deluded impression that all the guests at one of his parties were trying to steal his fortune. To prevent this he summoned the water spirits to destroy his guests. They were stopped with the combined efforts of Hearth and Jack.