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After intercepting the [[Death Guard]] ship ''[[Eisenstein]]'', Imperial Fist [[Primarch]] [[Rogal Dorn]] was informed that Horus led the [[Sons of Horus]], Death Guard, [[Emperor's Children]], and [[World Eater]] legions in rebellion after slaughter any loyalists within their forces. Dorn responded by creating a "Retribution Fleet" that would strike against the traitors and bring them to heel. The fleet, representing 1/3rd of the Imperial Fist forces, was set upon by turbulence in the warp and lost many ships before becoming becalmed in the [[Phall System]].

Unable to leave, the Imperial Fists were set into a routine of drilling and preparing for combat. After 141 days, the Iron Warrior fleet appeared and immediately proceeding to attack their hated rivals. The battle went poorly for the Iron Warriors; though they outnumbered the Imperial Fists, they suffered substantial losses. Before the advantage could be pushed by the Imperial Fist, communications from Dorn made its way through the storm and ordered all Imperial Fists to return to [[Terra]]. The Imperial Fists, loyal to their primarch, immediately departed from the system and suffered substantial losses.

Although the Battle of the Phall System seemed like a victory for the Iron Warriors, the Imperial Fists showed that they were a stronger force with an aptitude for battle tactics. The win was a bitter reminder to the Iron Warriors of how the Imperial Fists always overshadowed them, and the moment weighed heavily upon the mind of the Iron Warrior Warriors' Primarch [[Perturabo]].

After intercepting the Death Guard ship ''Eisenstein'', Rogal Dorn was informed of Horus's treachery at [[Isstvan III]]: he led the Sons of Horus, Death Guard, Emperor's Children, and World Eater legions in a purge of any legionary who was loyal to the Emperor before rebelling against Imperial rule. Dorn responded immediately by divided his legion in two. He would lead the Legion’s veteran companies to [[Terra]] to inform the [[Emperor of Mankind|Emperor]] of the betrayal unfolding. Meanwhile, Dorn ordered [[Sigismund]] to lead the remainder of the Legion as a "Retribution Fleet" to the [[Isstvan system]]. The Retribution Fleet was to reinforce the loyalist elements of the Traitor Legions and strike back at Horus.{{Fn|5}}

However, shortly before the fleet departed from the ''[[Phalanx]]'', Sigismund met with [[Euphrati Keeler]], who warned him of the cataclysmic [[Battle of Terra|cataclysmic battle]] to come on Terra. Sigismund decided that his duty was to remain at his primarch's side, despite Dorn's orders, and so requested that command of the fleet be given to someone else.{{Fn|3a}} Dorn acceded, and command of the fleet fell to Captain [[Yonnad]].{{Fn|3b}}

Under Yonnad's command, the Retribution Fleet was composed of approximately one-third of the Imperial Fists' warriors and fleet, numbering over 561 warships and 300 [[Space Marine Company|companies]] of Astartes .{{Fn|3b}}. These forces were augmented by [[Astra Militaris]] from the [[Sol System]] and elements of the Saturine Saturnyne Reserve Fleet.{{Fn|1}}{{Fn|4}}

To support Horus, the [[Gods of Chaos]] had afflicted the [[Warp]] with terrible storms to obstruct the [[Imperium]]'s response to Isstvan. The storms severely disrupted the Imperial Fist deployment by denying [[astropath]]s the ability to navigate and [[psychic]]ally communicate across long distances.{{Fn|1}} The storms were so severe that they destroyed approximately a third of the Retribution Fleet (reducing its strength to 363 warships){{Fn|3b}} during the attempted navigation to the Isstvan System. Fleet Master Yonnad was killed during the voyage and command over the Fleet was transferred to Captain [[Alexis Polux]], who was Yonnad's greatest pupil.{{Fn|3b}} Although there were more experienced commanders within the Fleet, Polux was considered by Yonnad to have a spark of genius and untapped potential. {{Fn|4}}

Ultimately, the Retribution Fleet was unable to find a way through the warp storms to the Isstvan System and lost half of its astropaths during attempts to communicate with Terra.{{Fn|3d}} But a break in the Warp Storm was discovered in the Phall System and Fleet Master Polux ordered the Retribution set up a temporary base of operations there lieu of risking additional mass attempts at warp transit. {{Fn|4}}

The Phall System consisted of two habitable [[agri world]]s, [[Phall I ]] and [[Phall II|II]]. Both were recorded as peaceful, loyal to the Imperium, and lightly populated.{{Fn|4}} Captain [[Amandus Tyr]], a veteran commander, favored favoured resuming aggressive attempts to navigate to the Isstvan system. Polux rejected this idea, but allowed Tyr to command ten ships to search for a break in the warp storms that would allow journey to the Isstvan system or communication with the Imperium.{{Fn|3d}}

As the battle continued, the Imperial Fists gained the advantage. Slower battleships drew fire while fast strike groups conducted hit-and-run attacks against the edges of the Iron Warrior cone-like formation, peeling back its layers of defense defence from the outside-in. These tactics were very successful, with one strike group led by Captain [[Amandus Tyr]] aboard the ''[[Halcyon]]'' achieving a kill ratio of 4-to-1. The effectiveness of the Imperial Fists defense defence has been credited to their high readiness, technological edge, and the performance of Fleet master Polux, whom history has recorded as being among the few Astartes who could rival - or exceed - the mental abilities of some primarchs.{{Fn|3c}}

As the Retribution Fleet was withdrawing, the Iron Warriors aboard the vessel ''[[Contrador]]'' commanded by Captain [[Erasmus Golg]] boarded Captain Polux's warship, the ''[[Tribune(Warship)|Tribune]]'', which was ultimately destroyed. But before the''Tribune'' was lost, Polux led a boarding operation of his own against the ''Contrador.'' The action was successful. Captain Polux killed Captain Golg in hand-to-hand combat and took control of the ''Contrador,'' broke away from the Iron Warriors armada, and joined the remaining Retribution Fleet on course for Terra.{{Fn|3c}}

*{{Endn|4}}: [[The Horus Heresy Book Three - Extermination]], Chapter "The Battle of the Phall"pgs. 46-51