Venerable Dreadnought Nihilus

lf a Venerable Dreadnought suffers a penetrating hit, you can make your opponent re-roll the result on the Vehicle Damage table.
You must accept the second roll , even if it is worse than the first.

The Mission Tactics special rule represents the unique way in which the Deathwatch go to war.  At the start of your first turn, pick one of the Mission Tactics.  It will remain active for the entire battle unless you decide to change it as described below.  As long as the Tactic is active, it affects all units in your army that have the Mission Tactics special rule.                                           
                                                                                                                                                             
Once during the battle, at the start of any turn after the first, you can choose to change the current Mission Tactic to another one.

Revered acient Nihilus has a 6+ invulnerable save, which is increased to 5+ against damage sustained from enemy witchfire powers.