Killer Clown addon replaces one of the mobs in the game at psi-clown. The most interesting part of this add-on is the model itself, which looks well enough. The killer clown really can not be called a boss, since he only has 10 hearts. If you are going to use it, I recommend you to install this addon together with the theme of Halloween cards. Clown killer replaces Wieser skeleton. It is equipped with an axe and inflicts blindness on their victims before killing them. Wieser skeleton can only be found in Hell. So you need to either go to Hell or use spawn eggs.