The folktale motif number is S241 - Child unwittingly promised
This is known from The Witcher as The Law of Surprise - when someone promises to give up "the first thing they see" when they get home, or "the thing they don't know they have". It is a very popular motif, with hundreds of tales.
If you are looking for something specific, I can help more :)
@yonah Personally, I know of a tale where a couple sold their firstborn to the Devil in exchange for ending their childlessness.
...I got the impression that they were not particularly bright.
Attached: 1 image The Manowick -- Part One (A Thread) In a village nestled among ancient woods, where whispers of forgotten sorrows lingered in the wind, a dark tale took root—the legend of the Manowick. Long ago, during a time of despair and affliction, a dreadful plague swept through the land, claiming countless lives and leaving behind anguished souls. Legend had it that the pit where the afflicted were buried became a breeding ground for vengeful spirits, merging into a grotesque entity known as the Manowick. Born from the twisted remnants of once-forgotten souls, it sought to unleash its wrath upon the living by poisoning their precious water. As the Manowick haunted the village, its wicked presence weighed heavily on the hearts of the villagers. Desperation consumed them, and they turned to a renowned sorcerer, known for his mastery over the supernatural, seeking aid in their plight. With ancient texts and forbidden rituals, the sorcerer delved deep into the secrets of the Manowick. He discovered that the creature's spell could be broken, but the price for liberation was grave indeed. It demanded the sacrifice of a firstborn—the innocent life to free the tormented. #folklore #ghost #ghoststories #folktale #mythology #MythologyMonday #writersofmastodon #AmWriting #WritingCommunity @[email protected] 1/