A fairy is a mythical being often described as a sort of spirit. Fairies are generally represented as humans in appearance with magical abilities, and it is said they would apply their magic to disguise themselves.
In a time when people thought that they could really meet fairies, it was generally accepted that these dark fairies were malicious and often dangerous. Tangling the hair of a sleeper, stealing small items, or bring a traveler on the wrong road, were considered harmless pranks. However, they were also charged with deadly conduct.
Consumption (tuberculosis) was considered to be a fault of the fairies. They forced boys and girls dancing every night, which made them tired for lack of sleep. Mysterious diseases of domestic animals were caused by dancing fairies.
In other legends, the dark fate liked to abduct humans. Both young men and women, or children, by swapping places and left. Ugly kids are fairy babies they are left in place where they have stolen human children. Even the older subjects might be kidnapped.