In the ancient ages when the humans reigned over Minecraftia and the mobs where hunted down.  
The End was still only a mith until a group of mages knew as the Arcaneus created a portal to the mythical dimension.  
They thought there could be new incredible resources that would help the mankind.  
They thought wrong, the inhabitants of the dimension was aggressive and hostile, some of the explorators were kidnapped and with the portal open the creatures could reach the Overworld.  
The humans that got kidnapped got infected by the dimension itself turning in strange abomination between the Endermans and the Humans, these abominations were hostile to everything that lived in the Overworld, there were many of this Abominations, 

this Warriors variant was the terror of any human unit, this extreme hard-to-kill-enders was completely covered in a thick armor, even the head was almost comepletely covered leaving only the holes to see.
They usually carried diamond enchanted axes to breack other armors and diamond enchanted swords.
they were extremely hard to kill but very slow and not agile at all, so most of the times it was better avoid then fight them, or trap them with an explosive or sand trap.

a war raged and both factions destroyed eachother, the remnants got then deleted by time.  
Only few things survived such has the Stronghold which was preserved by the arcane powers of the portal, the humans got almost completely wiped out except for remoted villages and few survivors scattered around Minecraftia that tried and still try to rebuild the ancient empire which was Minecraftia.  
The End didn't did anything better, all the Abominations died and only the Endermans remain them too scattered around Minecraftia unable to return to their homes due to the portal being closed, it's told that a human would be..strong enough to mix the arcane powers to create keys to open the portal again and kill the mind behind the End once and for all.