The ender dragon is a dragon which guards the End.
Confronting the Dragon
Gameknight and his group had to kill the Ender Dragon yet again, for they were in The End and the only way to escape was to slay the Dragon. Fortunately, they succeeded, but the Ender dragon respawned as Herobrine from the Dragon Egg at the end of the book.
Destruction of the Overworld
The Ender Dragon, as Herobrine, was terrorizing and attacking the villagers. However, Gameknight managed to kill it once and for all and the Ender dragon hasn't reappeared since.