Valids are a sanctimonious lot that consider themselves the true heirs to humanity, but they believe they have good reason to think so--other than their un-mutated appearances.
After the Cataclysm, humans on the surface did their best to survive. The first generation of mutants that rose from the disaster were beastly and made the survivors their prey. Those humans who hadn't sought shelter underground and not yet succumbed to mutation would fall to one of two fates: mutation or gruesome death by those who had.
Feeling helpless, the now subterranean Valids could only watch quietly from their
secure bunkers. They made sure that neither they--nor subsequent generations--ever forgot the horrible fate of their brethren topside. At least, the leaders of the pure strain humans made sure to use the atrocities to their advantage--crafting an impenetrable police state that gave them full power over the populous.
They focused their energies in building incredible underground cities and advanced technologies. In time they flourished, but they still needed resources from the surface so they organized salvage posses. When they re-emerged on the surface they quickly realized they'd technologically surpassed the topsiders and retained a "civilized society".
The Valids long buried their resentment underground, and now they finally had the means and opportunity to vest themselves of a centuries old contempt. Soon their salvage parties were bringing back mutants as prisoners. Valid scientists immediately saw the possibility of testing new advances on near-human subjects without impunity. All too quickly they began sending out more hunting parties, which reaped a bounty of prisoners--so many that they needed a place to stow them until needed.
Eventually the experiment became entertainment. Word of the Valids' cruelty spread quickly amongst the mutant tribes and soon it would be
their turn to bear a grudge....
Underground city illustration via Comic Book Cartography
