In the Battle-shock step of your opponent’s Command phase, if an enemy unit that is below its Starting Strength is within 12" of one or more HERETIC ASTARTES units from your army, that enemy unit must take a Battle-shock test.
Each time an enemy unit within 12" of one or more HERETIC ASTARTES units from your army takes a Battle-shock test, subtract 1 from the result.
Enemy units affected by this Detachment rule do not need to take any other Battle-shock tests in the same phase.