Mavka
These weird creatures are said to be something like fey undead—nature spirits (most often nymphs, dryads and naiads), affected by the influence from the Negative Energy Plane. They inhabit fields, forests, rivers and lakes (in regions with a long winter, mavkas spend the winter hibernating at the bottom of the lakes and quiet rivers, and then migrate to forests in summer). As fey creatures, mavkas are vulnerable to cold iron, and as undead, they can be turned by clerics or controlled by necromancers.
The navka looks like a very pale, but otherwise normal (if naked or wearing only a torn dress) woman. From the front. The nasty surprise comes when you look at mavka’s back—and see her bloated and rotting innards visible through the transparent skin. Though, at second thought, it may not be the best idea to come this close to a mavka in the first place. They try to lure defenseless berks, and when they come closer, mavkas attack. In battle, they strike with their bare hands, causing a tickling sensation—affected creatures lose their concentration and are slowed. Mavkas are more chaotic than evil, and their motivation for attacking living humanoids is not known. They don’t eat their flesh nor do they their drain life force—although sometimes they steal handheld items like hair combs to use them themselves. Sometimes, these creatures will spare their victims, and they almost always leave children alone. At the same time, mavkas can be as silly and whimsical as many fey are. They often play and dance in remote forest locations. Most fey and nature spirits, however are very suspicious of mavka and do not join them. Slavic powers, especially Perun and the Sea Tsar are quite antagonistic to these undead.
The most puzzling thing about mavkas is how despite them being undead monsters, they seem to not be destructive to the environment. Not just that, mavkas make forests and fields around them more fertile and bountiful. The undead fey often keep crayfish, catfish and other aquatic scavengers as pets. Some of them can also cast druidic spells like entangle. Finally, there’s a mavka population on Arborea, and the plane’s properties don’t seem to hold them back. Some graybeards have suggested that the mavka prove that death and decay of the Negative is just a necessary part of nature, and not inherently evil.
Mavkas are notably easy to control, and many necromancers make the mistake of taking advantage of this, and letting down their guard. However, once in a year—the precise week seems to be dependent on some unclear natural cycles—mavka becomes immune to any kind of controlling or charming abilities for approximately a week. Depending how the necromancer has treated the mavka, things can get messy. These stories sound very unlikely, as mavkas are very distressed by living in captivity and visibly rot away pretty fast when locked away from nature.
There are tales of “domesticated” mavkas, who liked living amongst the civilised folk and even started wandering the planes.
Stats: Mavka have been turned into CR12 creatures in the Pathfinder [PF1e] rules system here. Alas it’s behind a paywall, I can’t find a publically available version.
Sources: Margarita and Jon Winter-Holt. This is a homebrew addition to the lore. Margarita notes: While this adaptation is heavily based on Slavic folklore and beliefs, I must warn you. The amount of actual information we have on pre-Christian Slavic deities is so minuscule that building any kind of lore out of it is impossible. However, there’s a lot of folk beliefs about things these deities are thought to represent.