The Dark Cultists are tethered to an otherworldly being they refer to as the "All Mother". They scavenge ruins and dungeons looking for lost artifacts that may one day be used to prepare this world for her arrival. Of course it is just as likely they are being manipulated by a cabal of necromancers or some other nefarious organization.
Whatever the dark truth is, there is strength in their devotion. Each combat turn they survive renews their fanaticism and increases their attack rolls by +1.