The cultists are the most autonomous and industrious of the undead. They spent their lives practicing black arts and bowing to all manner of atrocity. Centuries of sleep has apparently done nothing to eliminate old habits.
Former primordial priests, banished magi, worshippers who sacrificed and were sacrificed. Who knows what else?
Something in that black soil nurtured their mystical talents, and inspired them to rise rejuvenated.
— Richard Sonnac's after-mission report on "They Never Stop Coming," The Secret World