Greek folklore has it that there are little, evil, hairy creatures named Kalikantzaroi (trolls), that live in the center of the earth and all year long using a big saw they try to cut down the tree of life that holds/carries the planet.
They are ugly and mischievous and they are afraid of the fire and the holy water. During the 12 days of Christmas they come up to the surface of the earth to bother and annoy people, playing tricks on them and causing trouble. On Epiphany Day (see related post) when the people and priest spray Holy Water they disappear immediately going back to the center of the earth only to find that the tree has fully grown back and they have to start all over again till next year!
These are stories grandmothers used to say to scare kids into behaving well during the holidays.