Rusalkas are fey creatures linked to particular lakes or rivers, who sing beguiling songs to lure companions to them. They are possibly related to sirens, but unlike those pernicious creatures, a rusalka isn't necessarily cruel, simply lonely and self-interested. They lure lovers to their bodies of water, and then impart the ability to live underwater to them. The two live together for a time before the rusalka inevitably withholds her gifts, forcing the air-breathing lover to flee.
Rusalkas generally don't engage in combat -- they flee, and can only strike weakly if pressed. However, they occasionally employ their song to lure aggressors to their own demise, drowning them or bringing them onto dangerous ice floes, or into the paths of more dangerous creatures.