Vodyanik and Antonin Dvorak
A water creature of Slavic lore. He drowns humans and sometimes makes them his slaves. With his Rusalka wife, he lives at the bottom of lakes. He catches and eats people who venture near the water. He appears as a merman or frogman, but is said to be able to change his shape to human form.
During the Romantic era when artists became more interested in integrating supernatural and mythological elements into their works, Antonin Dvorak wrote a symphonic poem (an orchestral piece meant to tell a story solely through music) based on the Vodyanik, or Vodnik.
In Vodnik, the titular creature kidnaps a girl and forces her to be his bride. They live in his underwater castle and have a child. One night, the girl begs the Vodnik to see her mother, and he allows it on the condition that the child remains with him, she can not embrace anyone, and she must return by evening. When she reunites with her mother, the mother pleads with her not to go back to the Vodnik and ends up forbidding her from leaving once the sun sets. The Vodnik knocks at the door, saying that he needs his wife to make him dinner, but the mother tells him to leave. He knocks a second time, saying that the beds need to be made, but again the mother tells him to leave. He knocks a final time, saying that the child needs to be put to bed, to which the mother simply replies that he should bring the baby to them. In a rage, the creature returns to his lair. Later that night, there is a storm and screams can be heard. Then, there is a loud crash, and the girl wakes up to find her baby outside the house, having been decapitated by the Vodnik and thrown violently against the door.
Dvorak also writes in a more benevolent Vodnik in his opera Rusalka, which is essentially the original Little Mermaid told with Slavic mythological figures, with Vodnik being the father of the Rusalka who falls in love with a human prince.
Wanting his daughter to be happy, it is he who points her to the witch Jezibaba in order to become human. The prince rejects her initially, and Jezibaba's spell causes him to become ill and Rusalka to become a demon. In the end the prince decides to die by Rusalka's kiss, and Vodnik , cursing humanity and grieving for his daughter's fate, states that all sacrifices are futile.
You can listen to Vodnik here and watch Rusalka here
Rusalka herself appears in Shin Megami Tensei and Persona along with other lesser known Megaten games as part of the Fairy clan.
Her function is essentially the Slavic equivalent of the siren or kelpie, living in water and enticing men and children to their deaths.