Cuckoos Nest

The myth of the Cuckoo's swapping childrens with darker or stranger creatures as been part of Godai for as long as they been stories told round the campfires in the distance past. The Cuckoo's are supposely a group of Fae from both the Light and Dark, that place their own children in place of those children they steal for who knows what.
Their is almost as many stories of the Cuckoo's Changlings as their are cultures within the world, Some talk about the fact the Cuckoo's pay for upbringing of their children, to those that are the true horrors and those changlings end up murdering their adoptive parents and bring in other dark creatures to replace them in the community and become somewhat of a honey trap to all around them, with more people going missing in the following years till their masks are lifted and the locals realise just what they are, and they have to run, hide or die.
The Cuckoo's of the myths probably do have some basis in reality, due to the fact that the Faewild and the Shadowfell time streams have some strange affects and growing up in them could take 100 years or 10 months, so the Fae could easily want to allow they children to grow up in the physical world and mature in a more normal and balance place. Before coming back to the Fae courts and learn who or what they really are.

Historical Basis

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Yes the stories are true the darker ones are probably more true than those of rewards and payment. The Fae see the people of Godai as toys and cattle at best, they use them for their own games and amusement, bring up their children is the least they can be expected to do

Spread

These stories are told though out the kingdoms of Godai, who knows what the truth is or if they are real, or just stories to make the People of Godai wary of the Fae and the Elves, becuase the TItans are worried about them. No one seems to know anyone who child has been taken it always the next village over, or my great uncles daughter.

Cultural Reception

A warning about the deceitful creatures of the Fae, all cultures seem to be as wary of them as the others. Which as heighten even more now the Elves are back.

In Literature

There lots of stories both children and adult horror stories about changling children and what happen to them and their families.