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The kaster is a young naked man with gems on his body from Fynen (Denmark) folklore. Common people would usually encounter him sleeping in fields on sunny days, his gems glittering in the light. They could walk up to him and take as many gems as they liked, but if even one of their hairs touched his skin he would grab them and take them to his home underground.
The kaster seemed to target priests more directly. A priest would be minding his own business when someone knocked on the door late on a cold night. He would find a young naked (or only dressed in a blanket) man and invite him into the warmth, but as soon as the candlelight hit him the priest would see the gems shining. The man would thank him for his kindness and say the priest could take any gem as reward, but it would be a trick. If the priest tried to take any the man would move, making the priest touch his skin and kidnap him. The only way for the priest to avoid capture was to say no and offer the man some food. After he had eaten the man would leave and be unlikely to return.
This was meant as a commentary on greedy priests who didn't help others out of the goodness of their heart, but only for payment.