Enderking Add-on for Minecraft PE adds boss into your game, he will called Enderking. If you want to meet him, you can find him inEnd dimension. As you know, in 1.8 MCPE version, addon creators can add new mob and do not replaced on another mobs. So, this new mob Enderking will not replaced any vanilla mob.
How to kill new boss?
I really recommend you, when you will have battle with boss, you should attack him with long range weapons. Boss will able to disapear and then teleport in another good place for him, be always aware. He can attack you with poison effect.
If you will see that Boss does not move, you should come closer to him and take Ender Crystal from yoru body ( just tap on him )
all screenshots you will see below this text
download this boss addonfor Minecraft Pocket Edition 1.8 +