Two new cultist foes come into the fight right at the end of round four. They come down the hallway from the same direction as the defeated ghouls.
Until then, the only two foes remaining are the prelate and Pritchard. Neither Caerth nor Maur can see the new attackers, as they are in the room with the brazier. All the other characters can see them once they arrive, assuming none of the characters try to follow Maur into the room.