In the soggy depths of Slavic swamps and bogs lives the Bagiennik, water demons and servants to Wada. Follow me as I take an extended look at this hydrophilic water devil; I promise we'll both learn something.
The Bagiennik lived in rivers and lakes, bogs and swamps, streams and creeks, and anywhere else it could fit itself that was also cold, wet, and slimy. They weren't generally feared by humans as meeting with them wasn't fatal, they did however emit a mucous-like substance from their nostrils that was said to be most painful should it touch human skin. This secretion wasn't a defense mechanism, rather a normal function of their endocrine system; like sweating.
According to legend the Bagiennik were beholden to Wada (which I can find no information on) or Pani Jezior (Mistress of the Lakes or Queen of the Underwater Meadows and apparently the same person as Wada but I can't be sure) and considered her their goddess.
A very small group of merpeople, Bagiennik never grow more than about the height of a ten year old child, and live in social groups in lakes and rivers in the same regions as Rusalki, though they refuse to share their territory with Rusalki, and vice versa. Incredibly vicious in their protection of their territory, Bagiennik swarm those they see as trespassing, and use small, needle-sharp teeth to tear them apart.
Despite this viciousness Bagiennik are sought as trading partners. The burning mucous they produce can be bottled and used in an assortment of healing potions. While Bagiennik oil is usually used to treat rheumatism, indigestion, and heart problems, it can be used, when combined with Dittany, Lavender, Valerian and Murtlap Essence, to heal mortal wounds.