I'd never come across this bit of ancient British folklore before today, finding it in a clickbait listicle, but it suggests a number of adventure hooks. Might even work for
Madness Dossier.
Green Children of Woolpit [from Wikipedia]
The legend of the green children of Woolpit concerns two children of unusual skin colour who reportedly appeared in the village of Woolpit in Suffolk, England, some time in the 12th century, perhaps during the reign of King Stephen. The children, brother and sister, were of generally normal appearance except for the green colour of their skin. They spoke in an unknown language, and would only eat raw broad beans. Eventually they learned to eat other food and lost their green pallor, but the boy was sickly and died soon after he and his sister were baptised. The girl adjusted to her new life, but she was considered to be "rather loose and wanton in her conduct".[2] After she learned to speak English, the girl explained that she and her brother had come from Saint Martin's Land, a subterranean world inhabited by green people.