After you defeat the Enderdragon, you May notice a new room in the stronghold. This room will have a new block, called the morgue door. This door is uncraftable and can only be opened with the skeleton key. The key is crafted like a shovel, with bones for sticks and an ender egg for the top. You will enter a room made of bedrock called the morgue. This room is filled with hostile mobs, but they won't hurt you. You will walk down stairs into a giant room and it will close behind you. Then text will play, saying, "I used to be a man, just like you. I loved, lived, and died. And, as you know, all men shrivel up, or as some may say, wither." then he will spawn and the hardest fight in the game will proceed. At the end, he will drop the wither skull, which goes on your head, is indestructible and makes all hostile mobs, except for enderman, peaceful, but has an annoying overlay.