Plavithen is known to other worlds as "The Oubilette." It is a place where antedeluvian eldritch horrors have been locked up and forgotten.
Ages ago, longer than anyone's reckoning, an ancient technological society fought a protracted war against an infestation of extradimensional elder evils.
They won, after a fashion. The elder evils and many of their minions lie locked in massive, ancient, ultra-tech vaults, sealed away by mysterious energy fields and held in suspension tanks, kept there by reactors that will last eons more and maintained by self-replicating nanobots and advanced data systems that reconstruct their own subroutines to avoid going haywire.
The civilization that created these wonders and shackled these horrors did not survive. The fight against the elder evils ravaged the planet and eradicated most of the populace. And so, on a desolate, irradiated, starving planet, elder evil was locked away and civilization perished, mortally wounded by the struggle with its ancient enemy.
And time passed.
Life re-emerged on the world and magic rose. And a new world, less advanced but just as alive, grew atop the buried bones of the old.
The world as it currently exists is a traditional fantasy setting with technology comparable to 1700s Earth, blissfully unaware of the world's history. Ancient sites lie in isolated places, untouched for millennia. But nothing lasts forever. The past is stirring, and the heroes of this age will rise to do battle with ancient evils that their forebears could only shackle.