Krampus is a unique type of Wesen, and the main antagonist of the episode "The Twelve Days of Krampus" of Grimm.
Krampus is well known and feared throughout the Wesen community, his stories dating back centuries. Being a sort of Santa Claus' evil twin, who arrives before Christmas to punish children he deems naughty, he beats them with a wooden switch, before grasping them and putting them in his sack, then taking them to the tallest tree and hanging them there until the 21st of December when he eats the children (and teenagers) before disappearing into thin air. He also always leaves coal behind.
Krampus arrived in Portland in 2013, he attacked two teenagers who stole presents, beating them he kidnapped on called Quin, but the other managed to escape him. Later he did the same to another teenager, who also stole a present. But was caught in the act by a witness, not caring he left and carried on his sadistic ritual, kidnapping all those he deemed naughty and hanging them from the top of the tallest tree in preperation to be eaten.
Investigating the case, Nick Burkhardt and Hank Griffen, became suspicous of Wesen after the witness described the evil Santa as wearing a monsterous mask. Talking to Monroe he filled them in on the legend of Krampus, and warned them of his evil habits. Following the legend they managed to find the forest, and using his superhuman hearing Nick managed to find the tree. However hearing Krampus approaching, Nick went to confront him, while Monroe, Hank and Bud freed his victims.
Suprising him, Nick and Krampus fought it off, Nick managed to incapacitate him long enough to free his latest victim. The fight continuing, Krampus began to get the upperhand, being super strong even by Grimm and Wesen standards. Grabbing Nick by the throat he tried to choak him to death, however at that moment triggered by his near death, Nick felt an after affect of the Cracher-Mortel venom he was infected with a few months earlier, tempoarily enhancing his strength to a much greater level, with a single punch he knocked out Krampus.
Hank and Monroe arrived and the three of them debated what to do, encouraging Nick to kill him as they could not take him in and they could not let him go. Nick was unwilling to just shoot him, and hestitade, finally convince he was about to, when Midnight past. On the stroke Krampus returned to human form and revealed to have no memory of anything, and being unaware he even was a wesen. It turned out that Krampus's changed during 3 weeks every year in relation to the season, in this state they became a monster, but the rest of the year, they were normally good people.
Unsure what to do, Nick recorded this important information in his Grimm Dairy, for future Grimms and they decided to turn the problem over to the wesen council as there was nothing else they could do about it.