Shadow Masters

The warriors of a Vanguard Spearhead slip through
the shadows, barely visible to the foe. Enemies blaze
away into the gloom with increasing their panic, their
shots flying wide as covert specialists encircle their
victims and prepare to level the killing blow.
Each time a ranged attack targets an ADEPTUS ASTARTES unit from your army, unless the attacking model is within 12", subtract 1 from
the Hit roll and the target has the Benefit of
Cover against that attack.