"Soldiers! Stand tall, for today you face the enemy of our New Imperium! They are a blight on this world, corrupt to the last man and damned for eternity because they will face us! Fight for the New Imperium!"
―Lord General Hakan Giray, rallying his troops.
The Conquest of Titan was a major war waged by the fledgling New Imperium to capture Titan, where the remaining loyalist Adeptus Mechanicus, Black Templars and Adeptus Custodes took shelter, and execute the last High Lord of Terra, Captain-General Zakon Kingbane, to secure its own existence. The war lasted for approximately 5 years as the New Imperium waged a war of attrition against the loyalist forces until they were unable to resist an invasion. The war ended when Alistair Crackham led the final charge against Grey Knights' Fortress-Monastery. However, because the New Imperium decided to wage a war of attrition to capture Titan intact and didn't bombarded it, the command squad of the Grey Knights managed to escape, through the help of Iron Hands, and formed the Imperial Remnant in Segmentum Tempestus, a result which made the New Imperium earn a grand enemy.
"I leave Holy Terra to return shortly after or die trying. Which one comes first doesn't matter."
―Captain-General Zakon Kingbane, before escaping from Terra.
During the climax of the Second Battle of Terra, an Imperial fleet managed to escape the Reformist forces by taking advantage of the confusion. The Reason why this fleet was then pursued by the reformist armada relentlessly was because Captain-General of the Adeptus Custodes, Zakon Kingbane, was aboard it. The fleet eventually managed to evade the reformist armada and took shelter in Titan, which was untouched by the warfare that nearly destroyed Terra. Titan welcomed the fleet and prepared for war when the news were heard. The newly chosen Supreme Grand Master and the Captain-General prepared a defense plan to hold Titan until the reinforcements from Krieg and Segmentum Tempestus arrived. However, their plan was destroyed because the war in Terra ended sooner than expected and the New Imperium was built, bearing the name of the Imperium only because of the great respect their leaders had for the selfless guardian of Mankind, who continued to sit on the Golden Throne and remained ever vigilant. However, no matter, how respectful they were, they reformed the core of the Emperor's divine will, his word was altered and many heresies followed these reforms, acts that angered many. Zakon was one of them and even he was surprised to see how many of his fellow Custodes had fallen under the influence of this "New Imperium".
"Your arrogance knows no bounds Alistair! You used to be one of us! Show some respect and help me. Give up your useless cause!" "Wicked eyes and wicked hearts deserve neither respect nor mercy. My cause is the Imperium and I will not give it up."
As a desperate push was made into the heart of the Citadel, Alistair, along with Enoch Ryder and the entire Exorcists command squad, were ambushed by a force of inquisitors and Zakon Kingbane himself. Believing that the execution of Alistair would dissolve this heretical "New Imperium", Zakon had enacted a risky plan and ambushed the High Consul right after he had left his command bunker to advance with his men. Alistair, even though no match for the mighty Captain-General of the Adeptus Custodes, was not unprepared. Without a single word, the Exorcists assaulted Zakon, while Alistair, along with his personal guard, "The Consul's Chosen" lashed out at the inquisitors.
"Sir! The Iron Hands' Fleet broke away from the siege force and enemy units are boarding their ships. Also, the Imperator Class Titans on Titan have stopped responding. Orders?" "What?! All ships, open fire at their location! This is treason!"
―Grand Admiral Chaka to the command personnel of Retribution, after the treachery of Iron Hands is exposed.