Yulemen, or Yule Lads, are a group of thirteen mischievous creatures of Icelandic origin.
Children of trolls or ogres Gryla and Leppaluthi. They are small, misshapen gnome-like creatures with various monstrous traits in the original folklore. Unpredictable in their actions, yulemen may either pose a serious threat or a mere annoying prank. Some of them aren't even interested in interacting with humans at all. They are indifferent in the suffering they might cause, and are mostly in for the thrill of causing harm.
Each lad has a distinct personality and has a name that corresponds to their nature. Gully Gawk, for example, found it most entertaining to just hide in gullies, while Window-Peeper was notorious for peeking inside windows and look for things to steal.
Yulemen are an old Icelandic legend. Descendants two human eating trolls, they originally had no Christmas context. Their stories were used to scare children into obedience. As each of yulemen had their own personality some of them were murderous towards cattle or even people, while others caused material harm by breaking, stealing or eating things.
Iceland prohibited the use of Gryla and her children as means to discipline children in 1746. From then on they shifted towards more benevolent role, dealing punishment for naughty kids and rewards for good behavior. Punishments didn't include loss of limbs or even life either, and worst that could happen was to find a rotten potato hidden in your shoe.