Do not make a wound roll or saving throw (including invulnerable saves) against a mortal wound – just allocate it as you would any other attack and inflict damage to a model in the target unit. Each mortal wound inflicts 1 point of damage on the target unit, and they are always applied one at a time.
Some attacks inflict mortal wounds – these are so powerful that no armour or force field can withstand their fury.
