Before the Dragon
Long ago (or not so long ago as the fabric of time is quite thin in the End) there sat a planet in an alternate dimension just barely outside our own. The name of this world is lost even to Legend but we now call it The End. There lived a race of masterful builders now called the Endermen. these beings held a society completely built around the idea of building. The measure of one's life was determined by the things one built from the tallest statue to the finest sculpture. Rather than enact war upon one another the Enderman settled their differences and gained their stations through great feats of construction. All Enderman could build, it was in their very souls and not a single one lacked the mind of an artist. They built up and up, further and further until the entire world was covered in structures that reached space. This became an issue for there was no longer room to build and without the capacity to build the Endermen fought amongst each other for dominance.
Soon the population of the End dwindled to nearly half of it's original number. Seeing the horrors of this war a young and headstrong Enderman took matters into his own hands and began to destroy the magnificent structures across the planet. The other Endermen were horrified at the sight of one of their own destroying these great monuments and so retaliated with violence. At the top of the grandest structure, they fought until the weight of so many angry Endermen toppled the structure. Upon crashing back into the planet the impact awoke a being from the core of the world. It released it's energy and imploded the End causing the planet to shatter. Some chunks flew into far space, some were destroyed completely, but the largest piece remained in the same spot, it's inhabitants left to face the vile creature...
The Ender Dragon
The creature known as the Ender Dragon destroyed the End and enslaved all the Enderman on the largest piece of the planet that remained. The Dragon could control any Enderman it could see and not one had the will to resist. It flies above what remains of the End commanding it's slaves to go forth into other dimensions and conquer in it's name. The Ender Dragon does not seem to be able to leave the dimension it currently resides in and must use the Endermen to conquer other realms. Why it even wants to conquer is a mystery as it's motivation and purpose is unknown.
The Endermen encountered in the modern day are the slaves of the Ender Dragon, they have lost their free will and sanity. Sometimes their builder instincts still show as they attempt to take blocks and stack them, but it is believed that they are too far gone to be saved. The Dragon has warped their minds and their form, making them far taller and removing their capacity for intelligent speech. They only know how to kill now but they cannot fight what they cannot see. Their eyes only see in an odd spectrum that shows the properties of the environment. For some reason looking into the eyes of another being makes them visible to the Endermen, perhaps Souls register in the spectrum or perhaps all creatures just look like dirt to the Endermen except for the eyes.
Those few Endermen who were on the smaller chunks of the End when it imploded managed to avoid the Dragon's control and so retain their original states. They can adapt quickly to new languages, build complex constructs, teleport small distances, and remove material from the ground without tools. They are also much shorter than their Slave counterparts and possess better dexterity. They are few in number and are rare in any dimension and are not hostile to those who are not hostile to them.