Underterror

From publisher blurb:

Infernal contracts are rarely to be lauded and though the gitwerc of Aventyr’s Underworld are already indebted to the devils of HEL, some take their agreement even further. These horrifying individuals morph their bodies to the extreme, using their unholy powers to infiltrate society before overwhelming enemies as they reveal themselves to be underterrors! This brand new base class and its archetype, the savant of HEL, are designed for use by gitwerc in the subterranean caverns of the Underworld of Aventyr, but their misbegotten powers are certain to bring a fearfully fun time to any Pathfinder Roleplaying Game!