Upiór

Within Slavic popular belief recorded in ethnographic and summary sources, the upiór is not a created monster so much as the outcome of a failed postmortem transition: souls normally depart and wander for about forty days before moving on to an afterlife, but where rites fail or death is sudden, violent, unbaptized, or ritually improper the soul can remain 'unclean' and return to animate or possess the corpse. The upiór category collects these cases—those who died cursed, by suicide, in childbirth, whose bodies were desecrated, or over whom animals jumped—and in some regional accounts the upiór tradition has close ties or overlap with other revenant categories such as the strzyga.

Descriptions associated with persons likely to become upiórs emphasize bodily anomalies or marks recorded in folklore: red hair, left-handedness, a limp, a unibrow, a double set of teeth, a gray mark on the back, an unusually large head, or absence of armpit or pubic hair. Corpses thought likely to rise were said to show signs such as lack of rigor mortis, a flushed face, or blood beneath the nails. Folklore sometimes describes upiórs as having doubled vitalities—'two hearts and two souls'—a trait noted in comparisons with the strzyga. They were reported to roam the neighbourhood, to be visible in evening reflections, and in some traditions to be active even in daylight although attacks were preferred at night.

Folkloric sources portray the upiór as a revenant form that sustains its reanimated body by harming the living, specifically by taking blood or otherwise causing illness and decline among relatives. Upiórs are described as vengeful or envious toward the living and as able to haunt family members, producing symptoms such as fatigue, pallor, sweating, and recurring nightmares in victims. Accounts emphasize that the unclean soul could re-enter or possess a decomposing body during the postmortem period and then roam cemeteries, household spaces, and environs—sometimes visible in mirrors at evening—and attack preferentially by night though daylight roaming is attested in the tradition.