Battle of Phall
The Battle of Phall is one of the biggest space battles of the entire Horus Heresy and, given that the war took place over an entire galaxy that's saying something.
The Contenders
On team Emperor we have the Retribution fleet of the VIIth legion under command of Captain Yonnad. The Fists were known to have have the largest fleet of all the Legions.
On team Horus we have Perturabo, Primarch of the Iron Warriors and his war fleet, which consisted of some of the biggest and most well armoured starships that had ever been built.
The coming of the storm
Following the flight of the Eisenstein, Lord Rogal Dorn heard about Horus' plan to crash the Imperium with no survivors and dispatched a third of his Legion and a bigger part of his war fleet to take Horus to task. This Retribution fleet was sent ahead of the Raven Guard, Iron Hands and Salamanders who followed shortly after. Unfortunately the Retribution Fleet was marooned in the Phall system since the Chaos Gods thought that odds of 7 v 4 weren't stacked enough in their favour. Yonnad unfortunately died during the warpstorms and command was thrust on one Alexis Polux. Polux was suspicious of the calm around the Phall system when everywhere else was fucked as far as the third eye could see and began rigorously drilling and training the men under his command, which turned out to be a very good thing when the fleet of the traitor IVth turn up.
The Battle of Phall
Maintaining a defensive position near a planet, the Imperial Fist ship groups began looping around a point in space to form a sphere of guns. Polux was right about the suspicious nature of the system and sure enough the main Iron Warriors fleet showed up to kill them. Unfortunately for the Iron Warriors, Perturabo thought that Sigismund would be in command of the Retribution Fleet. Sigismund was expected to do two things: land on the planet and build a fortress and charge at the Iron Warriors fleet guns blazing with the remaining ships. However, Sigismund decided to listen to a witch instead of his Primarch and was disowned and stripped of command of the fleet. Polux gave the Iron Warriors a much harder time than they were expecting. A boarding party got to Perturabo's throne room before being one-shotted and some of the ships got back to Terra... Oh wait that wasn't the Astronomicon, welcome to the Imperium Secundus.