Surreptus

 No one is entirely sure how the Surreptus came about.  Common speculation and
 rumor holds that overwhelming greed and the pursuant nature of Justice drove a
 band of thieves belowground, where they lived for decades, biding their time
 until they were forgotten.  As the years passed, fraternization caused their
 numbers to swell, their children developing to withstand the treacherous
 underground domains.  Little order existed in the society, and power was held
 by those who controlled what few material goods the group possessed, which led
 to incredibly violent skirmishes in attempts to gain them.  Over time, after
 the original generation had passed on, their history was forgotten.  Immolative
 magics used during the "Age of Vampires" eventually wore through the overland,
 revealing some of the tunnels to the surface.  As the magics continued to tear
 through vampire after vampire, a few Nosferatu stumbled onto the tunnels, and
 entered them, before sealing them off, thinking perhaps they were some
 forgotten entrance to their own familiar warrens.  These last of the Vampires
 finally found the then-childish race of underdwellers, jokingly calling them
 "Surreptus", noting their near-worship of  the wealthy items and thieving
 practices.  The Surreptus accepted the Nosferatu into their society as
 wise-men, learning of all the practices of the surface world, gasping in awe as
 they heard of the treasures to be found.  After the rumbling of the warring
 above ceased as the remaining vampires on the surfaces eliminated each other,
 below the surface, the Nosferatu taught the Surreptus to fight together towards
 a common goal, the value of fighting in packs, and usage of lightning fury on
 the unexpected, striking the unwary and unprepared before they become aware
 of what's going on.  With their training complete, the Surreptus venture
 upwards towards the surface world that is now so foreign to them.