"The Blemmyae in the forest"

"Don't go out into the forest alone! The Blemmyae will get you and eat you alive!"

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Blemmyae are not just a story, they are wicked creatures native to the highlands of temperate regions. Although distantly related to humans, some manner of magical mutation has caused them to be born without heads—their faces are set firmly in their chests. Blemmyae are raiders and scavengers, living off of the spoils of other civilizations rather than advancing their own culture.
Most blemmyae like to live in caves high on cliff faces, all the better to keep an eye out for passing caravans or travelers. Blemmyae may occasionally domesticate animals for meat and milk, but only those with as sure of feet as they, such as goats or sheep. Although blemmyae savagery is legendary, their disdain for technology renders them vulnerable to an organized defense. Most armor is useless without the chest covered, and many blemmyae consider the use of melee weapons a sign of weakness, preferring to feel the breaking of bones beneath their hard fists.
The blemmyae are not an organized people—their tribes are ruled by a simple doctrine of might makes right. Few blemmyae tribes are religious, but those that are are the most dangerous, as spiritual blemmyae prefer gods of elemental chaos and destruction. A blemmyae stands about four and a half feet in height but is stocky and muscular, weighing about 160 pounds

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Summary

The story is simply a go-to cautionary tale intended to scare children into not wandering of into the wilderness outside their home, and since Blemmyae are genuinely a threat in some regions they make an imposing and scary creature children would easily beleive to be in the area outside of towns that may not actually be threatened by the headless creatures. These stories dont even have to be Blemmyae, the creature of threat could always just be replaced with another. Although blemmyae are indeed an intimidating and alien choice.

Spread

These stories are pretty commonplace, especially in areas under threat of blemmyae tribes (although these stories are taken much more seriously in these areas), although there are plenty of people who use the threat just to get their children to behave.

Cultural Reception

In regions that actually have tribes of blemmyae in the area these stories tend to not surface due to the genuine threat they face in some communities.