Most Krevbornites are all too human—mortal creatures of passion and sorrow. They are a people of mixed ethnic origins who share a culture of folk tales and superstitions in common. Whether civilized or barbaric, they tend to face the horrors of their land stoically.
Some Greshnik are scaled and dragon-like, others are brutish, hunchbacked, or evidence horns of fiendish origin. They are not a separate people, but rather their unfortunate condition is commonly believed to be the result of witchery, misdeeds, or inherited sin.
The Polnezna are human vagabonds without a homeland who are adept at telling fortunes via the tarot. They either dress in colorful, exotic garb or the drab vestments of penitents, and travel in clans by wagon or riverboat.
Podmenysh are those whose ancestries bear a trace of fey blood. They sometimes appear completely human, but also may possess tell-tale features such as pointed ears, slight statures, ethereal beauty, or unnaturally colored eyes or hair.
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In a different system than D&D, Krevborna would probably be a humans-only setting. But in D&D the various races on offer do give a good bit of mechanical and thematic differentiation, so it would be a shame to lose them. Plus, some players really like having the option of writing "elf" on their character sheet.
Here, I've split the difference: broad-yet-folkloric ways of including the various races in the setting. Krevbornites and Polnezna give two variations on humanity; Greshnik encompass dragonborn, half-orcs, tieflings, dwarves, gnomes, etc. (with room for aasimar even, now that I think about it); and the Podmenysh fit-in elves, halflings, dwarves, gnomes, half-elves, etc.