The Merman's Children.
Vanimen bravely leads the mermen against the physical threat of the vodianoi but bawls and flees when he sees a rousalka/vilja, a woman who:
"'...haunts the water where she drowned.'" (p. 135)
Another character sympathizes:
"'...a ghost is terrifying in itself.'" (p. 136)
It is. If we believed that there was a serial killer in the house, then we would feel fear and, if we believed that there was a ghost in the house, then we would also feel fear but of a qualitatively different kind, inspired not by any physical threat but by a mere eerie presence. And we know what these feelings are like even if we do not believe in ghosts. This is a common inheritance from the times before there were electric lights and scientific knowledge. For previous relevant discussion, see here.