The Jester is a boss encountered at the end of The Kingdom adventure world in the Ruined Fort story dungeon. He attacks by throwing daggers and the occasional Jack-in-the-box which explodes when you're near. As a special attack, the Jester will split up into four different individuals and throw nothing but Jack-in-the-boxes at your minifigure.
The Jester steals the King's crown for Pieromancer Elementius and can be seen sneaking through the background of one of the cutscenes. The Jester then is found in the Ruined Fort giving the crown to his superior, Elementius. After you battle him, he is remorseful and asks for forgiveness joining in the quest.
Stay at a distance and use a ranged minifigure to attack him. When he splits into four different Jesters switch to a builder minifigure and build the quickbuild in the middle of the battle. The fake Jesters then float in the air, and proceed to crash into the ground and vanish with a purple explosion. After this, continue to attack by range and repeat.