The Cultist Missive is a tome that can be found on a lectern in the Temple of the Firstborn, in the blood ritual chamber where the player has to kill Blood Cultists and spill their blood in order to open the big door beyond (The Hematic Key event). It was written by one of the cult's leaders, whose name remains unknown. The missive reads as follows:
"He bade us find the favored children of Heaven and Hell and drain their blood. The children are few, and their strength is great. We take them unawares — unarmed or asleep — and open their veins. The blood flows into the temple, and our master smiles."