Bagiennik was the name of water demons in the Slavic mythology

Slavic mythology

Slavic mythology is the mythological aspect of the polytheistic religion that was practised by the Slavs before Christianisation.The religion possesses many common traits with other religions descended from the Proto-Indo-European religion....

. They were akin to the bathhouse spirit Bannik

Bannik

Bannik is the bathhouse 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...

. They were subject to Wąda, lady of the lake

Lady of the Lake

The Lady of the Lake is the name of several related characters who play parts in the Arthurian legend. These characters' roles include giving King Arthur his sword Excalibur, enchanting Merlin, and raising Lancelot after the death of his father...

s and the shallow streams, also known as the Queen of the Underwater Lawns.

The Bagienniks lived in the depths of lakes and rivers. They were capable of emitting an oily substance from the nostrils, which were located between the eyes or on the forehead. This substance was so hot that it burned the victims it hit, but it was also provided with notable healing abilities for rheumas, deep wounds, indigestion, heart illness and even infertility

Infertility

Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...

.

The presence of bagienniks could be detected by the bubbles on the water's surface, or by its darker and muddy colour.