The 7th Great Company were part of the Astartes task force assigned to secure Isstvan Extremis, the outermost world of the Isstvan System, ahead of the planned massacre of the Loyalist Astartes of the Traitor Legions during the Battle of Istvaan III. During this battle, Garro was badly wounded by the enemy leader who was a Slaaneshi mutant and he lost most of his right leg, necessitating an augmetic replacement once he was back aboard the Death Guard fleet