The Battle of Phall was a major void engagement that took place following the start of the Horus Heresy in the early 31st Millennium with the Istvaan III Atrocity, between the entirety of the Iron Warriors Legion's fleet and a formidable Imperial Fists Legion Retribution Fleet. At the command of the Warmaster Horus, Perturabo, Primarch of the Iron Warriors Legion, was sent to halt an encroaching Imperial Fists fleet that had originally been sent to Istvaan III by Rogal Dorn to reinforce the beleaguered Loyalists trapped on that world. The Traitors could not allow such a strong complement of Astartes to infiltrate their controlled area of space, for they would've seriously disrupted preparations for the Traitors' attack on Terra. Commanding the entirety of his Legion's war fleet, Perturabo