Krampus is the horrifying version of Saint Nicholas. He's one part demon, the other part goat. He's known to frighten children (or even beat them) so they end up on the nice list. But if children are bad, Krampus is happy to pull them down to hell, where he resides.
His name originates from the German word krampen, which means "claw." The German legend is that Krampus shows up on December 5th—Krampus Night—to see which children were bad before St. Nick comes later that month to reward the good children with presents.