At the start of the Declare Battle Formations step, select a number of ADEPTUS ASTARTES units (excluding TITANIC units) from your army based on the battle size, as shown below. Models in those units have the Deep Strike ability.
Each time an ADEPTUS ASTARTES model from your army makes an attack, if it was set up on the battlefield this turn, re-roll a Wound roll of 1. If it disembarked from a DROP POD this turn, re-roll a Hit roll of 1 as well.