There doesn’t seem to be a clear origin story for the Południca compared to other demons. Typically, (most) demons in Slavic mythology are the restless souls of those who lived “unnaturally” or died in violent ways. The tales I have been able to find tell of Południca being women who died within the fields beneath the midday sun – In this case, they are cursed to protect the field and avenge their death (which could’ve been caused by heat stroke or a bandit’s attack).
For most, the Południca’s presence was something to fear. Wielding a sickle or scythe, she would appear during midday, when it was typical for field workers to rest. It is no coincidence she comes at the hottest part of the day, as heat stroke was another of her weapons upon unsuspecting workers.
The Południca’s appearance is as frightening as her role. She is typically pictured as a young woman whose skin has rotted away beneath the sun. This sight of bones and decayed skin is oddly contrasted with her flowing blonde hair and the dress of a young woman, again, showing the curse that has befallen her. Romanticized modern art has made her appear more beautiful, but this fits the original folklore less.
Summarising: Don’t mess with Lady Midday or face the wraith of her sickle
(description taken from the website: https://brendan-noble.com/poludnitsa-poludnica-noonwraith-slavic-demon-of-noon-slavic-mythology-saturday/)