Chainsweep

Use this Stratagem in the Fight phase when an
IMPERIAL KNIGHTS model from your
army that is equipped with a reaper chainsword
or reaper chain-cleaver (or a Relic that replaces
such a weapon) is selected to fight.
Until the end of that phase, each time that model
makes an attack, you can never re-reroll that
attack's hit roll, but if that attack is made using
the Sweep profile of a reaper chainsword or
reaper chain-cleaver (or a Relic that replaces
such a weapon), make one additional hit roll.