Death to the Traitors!

Use this Stratagem in the Fight phase, when an
ADEPTUS ASTARTES unit from your army is
chosen to fight with. Until the end of that phase,
when you resolve an attack made with a melee
weapon by a model in that unit against a
HERETIC ASTARTES unit, on an unmodified
hit roll of 6 you can make one additional attack
against the same unit using the same weapon.
This additional attack cannot generate another
attack