Extremis trigger word

Use this Stratagem in the Fight phase, when an
ARCO-FLAGELLANTS unit from your army is
selected to fight. Until the end of that phase,
change the ability of all arco-flails models in
that unit are equipped with to the following:
Each time an attack is made with this weapon,
make 3 hit rolls instead of 1. At the end of the
phase, roll one D6 for each model in that unit:
for each 1, one model in that unit is destroyed.