Zel Tertius is a Frontier World in the Zel System of the Drusus Marches within the Calixis Sector. Recently, it has been the site of work by the arch-heretek Sar Resque, whose engineered mutating plagues have spread throughout the Marches. Mutants are upon the world of Zel Tertius, seen in the wildlands where there should only be loyal Imperial settlers of verified genes and purity. Imperial Guard veterans, deployed from the fearful hives, have discovered that communities are deserted, littered with contagious, twisted corpses and burned shrines. Mutants moan in the dark places beyond, tended dutifully by a pair of Transfigured twins. Deep within a cavern laboratorium, wizened biologis adepts and gun-servitors are doing the will of Resque.