The End is a dimension which has the Ender Dragon. It is a landscape made of End Stone. There are many Ender Crystals on Obsidian Pillars. Gameknight had been there twice- once in the Battle of the Source and defeating the dragon, and the other in The Second Time Battling The Dragon.
Events in the End
Gameknight999 and the Villager Army battled the Ender Dragon to get the Dragon Egg, one of the keys to The Source.
Feyd was crafted by Herobrine in the End, as well as Feyd's Ender horse.
Gameknight and the NPCs took Herobrine (in pig form) to the End to drop him into The Void, but he managed to infect the Ender Dragon at the end of the book.
Herobrine spent a brief time in the End as the Ender Dragon.