Leading the 4th Expeditionary Fleet, the Invincible Reason took part in many notable Imperial Compliance actions during the Great Crusade, as well as several major campaigns during the galaxy-wide civil war known as the Horus Heresy, including the Battle of Diamat and the Thramas Crusade in the Eastern Fringe.
It most recently participated in the Siege of the Fenris System in ca. 999.M41.
Subjugation of Sarosh
As the flagship of the 4th Expeditionary Fleet, the Invincible Reason took part in the Imperial Compliance of Sarosh (officially codified as Sigma Five-One Seven), an action which had formerly been commanded by an officer of the White Scars Legion.
The Saroshi -- ruled by a bureaucracy -- had recently expressed their interest in becoming part of the Imperium of Man, an interest the Imperium welcomed, as the Emperor's servants believed that the Saroshi possessed the same secular, materialist beliefs that defined the Imperial Truth. But the Saroshi secretly worshiped Chaos entities in the Warp they called the "Melachim," and saw the atheistic stance of the Imperial Truth as the embodiment of evil.
The Lord High Exacter, the leader of the Saroshi bureaucracy, denounced Jonson and the Emperor to the primarch's face aboard the Invincible Reason, and Jonson responded by ramming his power sword through the fanatical Saroshi leader's body.
The Saroshi had also brought a nuclear device aboard their shuttle, intending to assassinate the fleet's entire command structure, including Jonson; however, Luther and a junior Librarian named Zahariel El'Zurias managed to eject the shuttle into space, causing only minor damage to the flagship.
After this incident, Luther, Zahariel and five hundred other Dark Angels were sent back to Caliban in disgrace for allowing an enemy to get a nuclear device aboard the primarch's flagship. They were tasked with overseeing the recruitment of new Space Marines into the I Legion from the Calibanite population.
Thramas Crusade and the Imperium Secundus
The Thramas Crusade was a bitterly contested campaign between the Dark Angels and the Night Lords Traitor Legion that dragged on for nearly three standard years. Eventually, the Night Lords were defeated during a meticulously planned ambush by the Ist Legion, which decimated the Traitors' fleet and saw their primarch mortally wounded.
What little cohesion remained amongst the VIIIth Legion was broken. After recovering from his grievous wounds, the Night Haunter led an assault with his Legion's Atramentar Terminator elite in a desperate boarding assault upon the Invincible Reason. This assault resulted in the death of all but a dozen of the Night Lords Astartes.
Konrad Curze fled The Lion's wrath, and for a time took refuge within the bowels of the Dark Angels' mighty flagship. He continued to stalk the shadows of the vessel for solar months and unleashed terror and chaos amongst its mortal crew.
After the Dark Angels followed the psychic beacon of the alien device known as the Pharos through the dangers of the raging Ruinstorm, their fleet, led by the Invincible Reason, found itself within the Realm of Ultramar. At that time, the Night Haunter took his leave of the flagship and fled to the surface of Macragge, the Ultramarines' Legion's homeworld.
Lion El'Jonson eventually continued his obsessive pursuit of his elusive brother in an attempt to kill the Night Haunter and bring an end to his reign of terror. The Lion ultimately tracked Curze down and beat him to within an inch of his life.
The Night Haunter was then brought before two of his other brothers, Roboute Guilliman and Sanguinius, the Imperator Regis of the Imperium Secundus -- the second stellar empire of Humanity established at Macragge during the Heresy after communications with and transportation to Terra had been cut off by the Ruinstorm.
Once Curze was sentenced to death, the Lion once again intervened on his behalf. Requesting that his brothers stay their hands, he offered to be the Night Haunter's personal gaoler. The other primarchs agreed, and Curze was ferried back to the Invincible Reason, which became his prison once more.
Following the abolition of the Imperium Secundus, the Invincible Reason was among the vessels that served in the Loyalist fleet as Roboute Guilliman and Sanguinius moved through the Ruinstorm. They hoped to get through the Warp and reach Terra in time to aid in its defence from Horus' assault.
However, upon arrival at the cursed world of Davin, Sanguinius boarded the Invincible Reason and took the captive Curze with him for his own pilgrimage to the surface of the planet, knocking several Dark Angels guards unconscious so he could take his brother.
During the subsequent Second Battle of Davin the Invincible Reason became engaged in a vicious duel with the former Iron Hands Strike Cruiser which had been transformed into a Daemonship, the Veritas Ferrum, controlled by the daemon Madail.
Fall of Caliban
In the aftermath of the Horus Heresy, the surviving Dark Angels eventually returned to their homeworld of Caliban, which had been enshrouded by Warp Storms since Horus' betrayal. Upon the Dark Angels' arrival in orbit, their fleet -- led by the Invincible Reason -- was fired upon by the planet's defences from its surface.
The Dark Angels discovered that their garrison commander Luther -- who had become embittered by what he perceived as El'Jonson's penchant for always seizing all the glory of their campaigns together for himself -- had poisoned the minds of the Dark Angels garrison and the new recruits left on the planet against their primarch.
During the confrontation that followed, the Dark Angels' homeworld was torn apart during a horrific Warp Storm unleashed by the Dark Gods to protect the Traitors. It is believed that during this conflict, the Invincible Reason was seized by the Fallen.
After the destruction of Caliban, the Invincible Reason remained a part of the Dark Angels fleet. It is, at present, the flagship of the Dark Angels Chapter fleet.
- Descent of Angels (Novel) by Mitchel Scanlon
- The Primarchs (Anthology) edited by Christian Dunn, "The Lion," (Short Story) by Gav Thorpe, pg. 225
- Ruinstorm (Novel) by David Annandale
- The Unremembered Empire (Novel) by Dan Abnett
- Legacy of the Wulfen (Anthology) by David Annandale and Robbie Macgiven, pg. 489