Idea: End and endermen story
In an age long past and forgotten, there was a secretive and reclusive sect of mages who researched and hoarded knowledge of all kinds. Their libraries were so vast that librarians of today would be boggled at their scope. Just as all other isolated communities have experienced, these mages eventually vanished from the world. Legends used to whisper of portals to other worlds or of infestations of silverfish that killed everyone. Nobody ever learned what happened to them, but their underground strongholds where somehow sealed and lost to the rest of the world. The truth of what happened is far stranger and more tragic than anyone could suspect.
These mages did indeed discover a way to travel to another world, and in their fear of ignorant judgement from the rest of their world for their pursuit of arcane knowledge they abandoned their underground strongholds in their home world to create a civilization of their own in the new world. It was the neglect of their absence that allowed the siverfish to nest in the strongholds and seal them against invasion from nosy explorers. The mages were pleased to have the way sealed behind them, so they allowed the siverfish to remain while they developed their new home.
The mages grew in number and built towers across the world where each of their clans could pursue their own studies in peace. One of these clans studied the flora of this new world and found truly amazing properties in one particular plant: chorus. Eating the fruit of this amazing crop gave the eater the ability to teleport once. This discovery soon consumed all of their studies and quickly spread across the world. Teleportation without portals was beyond anything they'd ever imagined possible and must be pursued at all cost now that it had been found. With an entire world's resources bent on the study, it was only a matter of time before they learned to imbue this ability into the pearls they created for their portals and somehow make those pearls a part of themselves. Unfortunately, in their blind pursuit they missed hints of what was happening to the world around them and to themselves.
Teleportation of this nature has its costs. The chorus plant is more than just a plant; it's a dimensional parasite that feeds on the essence of reality. Reproducing in themselves the ability to teleport tied the mages to this parasite, and every teleportation caused the chorus that they'd spread everywhere to drain more vital essence from the world and its inhabitants.
Generations of mages abusing this power caused their minds and the world to break apart, exposing the void of insanity and unreality. The mages began to act more and more like the parasites they were becoming from their ties to the chorus: feeding off the world for their own desires. Since teleportation was limited in range, they needed a way to cross the widening void rifts between their settlements. They turned on the dragons they'd lived peacefully beside to steal their power of flight. The mages hunted dragons to near extinction to build flying ships and study their flight for their selfish pursuit of a means for individual flight.
The last dragon foresaw her doom at the hands of these insane madmen and appealed to the last remaining parts of their intelligence to prevent this disaster from befalling another world. She knew of the fear they harbored (which madness had only magnified) for the people of their original world and offered to guard the portal site against invasion. They agreed to let her live for this purpose and constructed pillars topped with crystals of life to sustain her for all eternity as the Guardian of their world. She forever since used this pretense to guard the way between worlds and ensure no chorus could ever escape her world's End.
There's nothing left now of the mages except husks of men still petrified of judgement and quick to anger. People have come to call them endermen for their violent wrath against any who even look at them, but the name is more appropriate than any know, for these men are enders of worlds, not just of men. Some small part of them still understands that their world is dying, so they steal bits of other worlds to try to repair and bring life back to their own, but their insanity prevents them from remembering what they are attempting and they drop these bits in odd places when some other thought crosses what's left of their void-warped minds.
Their world is doomed, and they are doomed with it, for once their world is consumed so will end the chorus they draw their power from. The few left scattered across their old world and whatever other worlds they've managed to find will be stranded, lost in their madness, exposed to the elements, and hunted for their extremely valuable teleportation glands. That is, unless something breaks the vigil of the Guardian...