Shadows

Appearances.
In episode: King of Shadows, Kai runs after a false Nya, (Lord Garmadon in disguise) into the Fire Temple, Lord Garmadon persuades Kai to remove the sword of fire or Nya will fall into a pit of fire. Kai removes the sword uses Fire Spinjitzu and cuts the chain holding Nya and sets her free. Kai's Shadow gets red eyes and starts attacking him. Lord Garmadon makes more Shadows appear, who keep attacking Kai until he drops the Sword of Fire. One shadow picks it up when suddenly, Sensei Wu, using his shadow, attacks the shadows and using his hands makes an eagle, which scares away the Shadows. Later in "The Green Ninja", Kai mentions that Lord Garmadon "Cheated" by using his "Shadow".

How to defeat them

 * Once you encounter a shadow, they're aforementioned as invincible entities. The only possible way to defeat them is to actualize your own shadow, and consider a way of defeating them using your own shadow.