Battle of Trisolian

The Battle of Trisolian
Date	~011.M31
Scale	Void battle
Theatre	Horus Heresy
Status	Imperial retreat
Belligerents
Space Wolves	Sons of Horus
Commanders and Leaders
Leman Russ, Belisarius Cawl	Horus
Strength
~40,000 Marines, the near entirety of remaining Space Wolves; Mechanicum loyalists	~50 capital ships; Sons of Horus Legion; elements of Word Bearers, World Eaters, Alpha Legion, Night Lords & Iron Warriors; Mechanicum Traitors
Losses
Extremely High, several companies reduced to less than 20% strength	Not known
Outcome
Effectively ended the Space Wolves as a "Legion", who retreated to Yarant. Horus badly wounded/partly de-corrupted.

The Battle of Trisolian is the ill-fated attempt by Leman Russ to kill Horus before the inevitable Siege of Terra. It also marks the earliest appearance of a young Belisarius Cawl who is a mere up-and-coming but still notoriously unconventional tech-adept by this stage.

Background

Late in the Horus Heresy, the Space Wolves legion had lost around 25% of their forces at Prospero and had taken a further maulings at the Battle of the Alaxxes Nebula, coupled with engagements at Vanaheim and Daverant, reduced their number to around forty thousand marines. Following Alaxxes the remnant returned to Terra to coordinate with the loyalist forces.

Leman Russ sponsored a stealth mission by the Knights-Errant on the Vengeful Spirit during the Battle of Molech in order to gauge any weaknesses in the traitor dispositions and report back on the state of Horus himself. However, the mission went sour when they were discovered, leading to the deaths of several of the Knights-Errant involved when the Warmaster decided to make examples of them.

Following the completion of this mission, Sanguinius and Jaghatai Khan had also arrived on Terra and convened a war council to determine the next course of action: Rogal Dorn urged that all remaining forces maintain a defensive posture, but was appreciative that the Ultramarines and Dark Angels were assaulting the traitors' rear and providing relief to loyalist forces, allowing them to dictate the next few battles. Dorn declared that Terra was too confined a battlezone for the loyalist contingent of Titan Legions and so proposed a muster at Beta-Garmon and make a stand there in order to give time for Roboute Guilliman and Lion El'Jonson to crush the traitors in a pincer movement. The Battle of Beta-Garmon would itself become the site of one of the greatest battles in the entire Horus Heresy.

However, Leman Russ would not be swayed from his original plan and chose to depart from Terra for Fenris in order to scry a weakness in the superhuman upgrades that Horus gained at Molech, determining that he should take the oft-maligned "Spear of Russ" with him.

The Battle

The battle itself took place over the Forge World of Trisolian A-4, which had recently been captured by traitor forces following a surrender of Mechanicum forces, although a loyalist faction of Skitarii led by Cawl continued to fight against the traitors.

The Space Wolf fleet used the unique gravitational properties of the three stars of the Trisolian star system to approach with stealth and speed, using most of their ships as a distraction while Russ own flagship engaged the Vengeful Spirit directly, allowing him to board with thousands of marines.

Russ had already figured that the battle would result in a loss for his forces, but thankfully Cawl's loyalist Mechanicum elements on the Forge World managed to overthrow the regime and revert the planet back to loyalist control, allowing it to lend a hand in the conflict.

Aboard the flagship; Russ engaged Horus directly and managed to score a wound with the Spear and allow Horus a moment of clarity where his brother would plead with him to rejoin the Emperor's cause. Horus still rejected the offer and the fight continued until Russ was beaten and defeated, though Horus was prevented from delivering the killing blow by a mob of hundreds of Space Wolves who literally dogpiled him, allowing Bjorn to drag him from the battlefield and back to the Loyalist fleet.

Aftermath

The Space Wolves were mauled further still, with some companies being reduced to less than a fifth of their already diminished forces, this effectively ends the existence of the VIth "Legion", though Horus ordered his forces to hunt down their remnants anyway and return Russ's head to him.

With their Primarch unconscious from his injuries, the remaining Space Wolf commanders have to make a decision of whether to stay together and likely die, or disperse and play no further part in the Horus Heresy. Choosing to remain together to present a viable threat that the traitors would have to divert resources to deal with, they retreat to Yarant III to make a stand where they are hunted by Failbaddon but are eventually saved by Corax following his own gene-seed debacle.

Horus was partially cleansed of the corruption he gained at Molech after being wounded by the Spear of Russ and a momentary glimpse of the original Primarch is witnessed by his brother, only for him to be corrupted back to Chaos soon afterwards. Horus would later fall into a coma at the aforementioned battle at Beta-Garmon as his wound reopened and a shard of his soul would be fought over by the Chaos Gods, requiring the sacrifice of Maloghurst in order to free him.