Rusalka/Rusalki

The Rusalka is a form of Slavic myths of Russia. They’re amphibious creatures thought to be the tortured souls of unbaptized children, babies downed by unmarried mothers, drowned virgins, and those who committed suicide out of heartbreak. They live at the bottom of lakes and can come to the surface and turn into humans by taking human clothes. They have legs on land, but only a fishtail in water. Some stories say the Rusalka lure in human men to bring them to an underwater palace but the men usually drown. Some stories have the Rusalka carry off men to a magical location to seduce and keep young forever with magic, however, when the person returns to the human world, they die instantly when their age catches up to them. Other stories said the Rusalka are predatory creatures that hunt children and drain their life essence. A cross made in the air can keep away the Rusalka and they can also be repelled by a cross made from a tree grown in a churchyard.

Citations:

Maberry, Jonathan. Vampire Universe: The Dark World of Supernatural Beings That Haunt Us, Hunt Us, and Hunger for Us. United States, Kensington Publishing Corporation, 2006.