Slavic Mythology I: Water Spirits, Demons, and Creatures

Rusalka by Ivan Bilibin
A rusalka is a female ghost, water nymph, succubus or mermaid like-demon in Slavic mythology that dwells in waterways. The plural is rusalki or rusalky. 
There are many different variations of how one becomes a rusalka, but they are always women (usually virgins) who died before their time near water. They become rusalki either because they are unclean souls or souls who suffered violent deaths.  Unclean souls could be those of unbaptized babies, babies born out of wedlock (and drowned by their mothers) or suicide victims.  Souls who suffered a violent death could be those who were murdered or committed suicide, these deaths usually had something to do with the woman being betrayed by her lover. 
After they have died they will haunt the water which they died in or near until and they will finally be able to rest when their deaths are avenged.  Unclean spirits have to live out a designated amount of time on earth before they can be allowed to rest.
They often come out of the water, climb in a tree or sit on a dock, and sing or comb their hair. Sometimes they join together and do a circle dance in a field. She either has green or golden hair, which always had to stay wet, because if it dried out she would die, for this reason she could not spend a large amount of time on land. She could however, stay on land if she had her comb because it provided her with the water she needed. A rusalka's eyes either shine like green fire, or they are extremely pale with no pupils.
They will either be malevolent or very gentle and playful, usually depending on where they are from.  People of different areas have assigned different personalities to the rusalki.  For example, around the Danube River, where they are called vile or vila (vila are also the Slavic version of nymphs), they are considered to be gentle, beautiful girls dressed in robes of mist who sing bewitching songs to the passerby.  However, the rusalki of Northern Russia were considered unkempt and hideous, and they would not pass a chance to ambush humans.  Rusalki vary from region to region, in Ukraine they were linked with the water, but in Belarus they were linked with the forest and the field.
When they are malevolent, rusalki crawl out of the water in the middle of the night and fascinate handsome men with songs and dance, then lure them into the river where they would drown, or tickle them to death, or they would die in her arms. She would lure children into the water with baskets of fruit.  In some variations, hearing her laugh could kill you.  When they are not malevolent, they enjoy having men and children joining in their games.  They will come out of the water to sing and dance.  But all rusalki love to entice men, either to enchant them or torture them.
Rusalka week is a week in early June which the rusalki were believed to be at their most dangerous. At this time they leave the water and swing on branches of birch and willow trees at night. They would dance in circles and any human who joined had to dance until they died.  Swimming was forbidden at this time, lest a rusalka would drag them into the water and drown them. A common feature of the celebration of Rusal'naia week was the ritual banishing or burial of the rusalka at the end of the week, which remained as entertainment in Russia until the 1930s. The word rusalka originally referred to the dances or girls at Whitsuntide. The word is derived through Greek ῥουσάλια from "rosalia", the Latin term for Whitsuntide week (originally it meant "the festival of roses").  After rusalka week, the grass grows thicker where they trod.

Bagiennik are water demons in Slavic mythology that are akin to the bannik (see above) and lived in the depths of lakes and rivers.  Their presence could be detected by bubbles on the water's surface or by its dark, muddy colour.  They were subject to Wąda, lady of the lakes and the shallow streams, also known as the Queen of the Underwater Lawns (I cannot find any information on Wąda, if anyone has information please tell me).  
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.

Topielec (Vodník or Utopiec) spirits of the water in Slavic mythology.  The topielec are spirits of human souls that died drowning, residing in the in the water which they died. They are responsible for sucking people into swamps and lakes as well as killing the animals standing near the still waters.

Vodyanoy is a male water spirit in Slavic mythology.  They are, in a way, the male equivalent of the rusalka (see above).  The East-Slavic and West-Slavic conceptions of this creature are quite different.  
Vodyanoy the Water Sprite by Ivan Bilibin
In East-Slavic mythology a vodyanoy is usually given the nickname "grandfather" or "forefather" by the local people.  Fishermen, millers, and also bee-keepers made sacrifices to appease him.  He looks like a naked old man with a greenish beard and long hair. His body is coated in black scales as well as algae and he has a fish's tail. His hands are more like webbed claws and his eyes burn like red-hot coals.  A vodyanoy floats around on a half sunken log, making loud splashing noises in the water. 
When a vodyanoy is angry he drowns people and animals, and he breaks dams and water mills.  When he dragged someone into the water, if he did not kill them he would make them work in his underwater dwelling as slaves. 
In West-Slavic mythology this creature is called a vodník.  He has a completely human constitution and habits, except for a few differences.  He has gills, webbed hands, green skin, and pale green hair.  It is not uncommon for a vodník to have a long, messy beard.  His clothes resemble a vagrant, and he wears odd hats, usually boater hats.  
They can remain out of the water for a long time but they will still be completely soaked.  
There are good and bad vodníci in West-Slavic mythology.  Depending on which they are, they may not drown those who enter their territory.  They keep the souls of those they have drowned in porcelain lid-covered cups, and they love to display them.  The number of cups is seen as wealth to vodníci.  When the lid is removed the soul comes out in the form of a bubble and is liberated.  The only servants that vodníci have are fish.
They generally spend all of their time running their territory or just chilling, by playing cards, smoking pipes, or just loitering at the waters surface.