The Slavic spirit said to live in a bathhouse and become dangerous when angered is the Bannik.
The bannik is a bathhouse spirit from Slavic mythology, especially associated with the banya, a heated communal or household bathhouse resembling a sauna. Folklore describes him as a small, naked old man with a long beard, sometimes covered in birch leaves left from bath brooms. He is also said to shapeshift into people, stones, or pieces of coal from the bathhouse stove.
The banya was viewed as a powerful and liminal place: it could be used for bathing, childbirth, and divination, but it was also considered spiritually dangerous or “unclean.” The third firing, or the fourth in some traditions, was reserved for the bannik. If disturbed, he might pour boiling water over an intruder, strangle someone, or invite other spirits into the bathhouse. People left water, soap, or fir branches to keep him peaceful. The bannik is distinct from the domovoi, a household spirit, and the leshy, a forest spirit.