In your Shooting phase, just after selecting a target for this model’s Stormsword siege cannon, roll one D6 for the target unit and every other unit within 3" of that unit: on a 5+, the unit being rolled for is struck by a concussive wave. After this model has finished making its attacks against that target unit this phase, each unit struck by a concussive wave suffers D3 mortal wounds.

While this model has 1-8 wounds remaining, subtract 4 from its Objective Control characteristic and each time this model makes an attack, subtract 1 from the Hit roll.