Rusalki are Slavic water spirits, similar in nature and action to sirens or kelpies—they are alluring, but potentially very dangerous, due to their compulsion to lead people into the river and drag them down to the bottom.
While there are folkloric comparisons to nymphs and mermaids, these can be a little misleading, because a rusalka is basically a ghost. Typically young, and always female, she is the left-behind spirit of a girl who died violently at the hands of someone else, or through suicide, and must fulfill the rest of her alloted time on earth. While the rusalki are beautiful and translucent-skinned, with long, pale hair and pleasing voices, they can easily be identified by their milky, pupil-less eyes and their perpetually wet hair.
They are typically most dangerous during early June, when they leave the water at night to climb birch trees and join each other in circle dances, and during the June celebration of the Green Week, swimming is forbidden in Russian villages, as the rusalki would like nothing better than to add to their collection of drowning victims. Children make particularly easy targets and it behooves Russian mothers to keep a close watch on their offspring.
In order to banish the wandering rusalki back to the water, village girls will typically construct and decorate an effigy woman of straw or birch branches, which will be thrown into the river as a reminder that the rusalki are not wanted on land and must remain in the water until next year.