Bannik is the bathhouse (banya) spirit in Slavic mythology. Slavic bathhouses resemble saunas, with an inner steaming room and an outer changing room. A place where women gave birth and practiced divinations, the bathhouse was strongly endowed with vital forces. The third firing (or fourth, depending on tradition) was reserved for the Bannik, and, given his inclination to invite demons and forest spirits to share his bath, no Christian images were allowed lest they offend the occupants. If disturbed by an intruder while washing, the Bannik might pour boiling water over him, or even strangle him.
The Bannik had the ability to predict the future. One consulted him by standing with one's back exposed in the half-open door of the bath. The Bannik would gently stroke one's back if all boded well; but if trouble lay ahead, he would strike with his claws.
See also 
- Alexinsky, G. Slavonic Mythology in New Larousse Encyclopedia of Mythology. Prometheus Press, 1973, p. 287-88