What is "kallikantzaros"

The kallikantzaros (, pl. καλλικάντζαροι kallikantzaroi; ) is a malevolent goblin in Southeastern European and Anatolian folklore. Stories about the kallikantzaros or its equivalents can be found in Greece, Bulgaria, Serbia, Bosnia and Turkey. Kallikantzaroi are believed to dwell underground but come to the surface during the twelve days of Christmas, from 25 December to 6 January (from the winter solstice for a fortnight during which time the sun ceases its seasonal movement).