hello friend. a mavka, navka or nyavka is an undead forest spirit, one of type which ukrainians call the covered dead (заложні мерці) — spirits of people that died of unclean, improper death, and therefore couldn't finish the transition and weren't allowed to the orherworld. the name covered undead comes from the fact that in old times they weren't properly buried, and were left in forests covered by leaves and twigs.
generally mavky are envisioned as beautiful girls, although in some regions there are beliefs about male mavky (sometimes called didky). mavka looks perfectly human except for the fact that their backs lack skin and muscle, exposing their innards and spine. they aren't malicious, but are obliviously playful and can hurt people during their plays — tickle or dance you to death, drown, ward you off your tray. in some western regions it's also believed that time goes faster when encountering them — what is felt like several hours could actually be several hundred years.
navky live in forests and mountain caves, and they like to dance, weave, play and prank wanderers, especially young men. to ward off mavky valeriana, garlic and wormwood are used, as well as wearing your shirt inside out. like most undead spirits in ukrainian mythology, navky are most active during the green festival/rusalka week.
there is a ukrainian holiday called navsky velykden, or undead easter, celebrated at the first thursday after easter. in this day all mavky, rusalky, upyri and all the other unclean forces celebrate easter. at night during the holiday it was prohibited to visit churches, since celebrating undead could dismember you if spotted.