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; majority of fighting forces on a sizable loyalist Forge World	~50 capital ships; Sons of Horus Legion; considerable 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. Unknown number of Mechanicum loyalists, presumably high, though a fair amount managed to survive the end of the war	Higher number & percentage than of the Space Wolves, but with sufficient reserves to absorb the losses largely unphased
Outcome
Effectively ended the Space Wolves as a "Legion", who retreated to Yarant. Horus badly wounded/partly de-corrupted. Leman Russ badly wounded and/or traumatized.

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.

Russ really wanted Horus's wolf pelt

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.

Once aboard the flagship, Russ engaged Horus directly in an extremely savage duel and incredibly, managed to best Horus in single combat, scoring a debilitating wound with his spear and literally (not figuratively) bringing Horus to his knees. Russ' spear had been made by The Emperor himself, and so contained some of his power, and as everyone here knows, that power is anathema to Chaos, so when Chaos-juicing Horus was wounded by it, the effect was even more destructive than it might otherwise have been. In point of fact, this power was so pronounced that when Russ began to further run him through, Horus was shown to visibly terrified, not by Russ, the wound, or even the very real prospect of death at the hands of his brother, but of the spear's ethereal power coursing through him, obliterating the effects of his chaos-juicing, and wrecking his massive gains. Unfortunately, Russ was then suddenly and without warning beset by a rampaging plot device by way of narrative cliché that momentarily overcame his better judgment and stated goal. Thus, instead of immediately killing Horus like he should have done (that was precisely the primary mission objective remember) the wound gave Horus a brief moment of clarity, and so despite the sacrifices made of himself, his sons, and some of his closest advisors (like the death of his chief Rune Priest, Kva), not to mention having heavily negatively impacted Dorn's defensive plans for the upcoming siege by leaving Terra, and that the single most powerful motivator for Russ had been that he wanted to protect his father, now, with the Arch Traitor himself at his mercy, Russ instead tried to plead with his brother to rejoin the Emperor's fold. Horus declined and then suckerpunched Russ, starting a new duel. Horus was about to kill him, but was prevented from delivering the killing blow by a mob of dozens of Space Wolves who literally dogpiled him, allowing Bjorn to drag Russ from the battlefield and back to the Loyalist fleet where they began to disengage and retreat.

Aftermath

Already heavily depleted from all the military actions since Prospero, following Trisolian, the Space Wolves were mauled further still, with some companies being reduced to less than a fifth of their already diminished forces, leaving their effective fighting force to somewhere in the range of 10 to 15 thousand marines. While that number may still sound a tad higher than expected, consider that A, this is all that they had to get through the rest of the war and the subsequent Scouring, B, they would eventually form an ill-fated Second Founding chapter, also made largely of veterans of the Heresy. In any case, without any means of quickly replenishing their forces, this effectively ended the existence of the VIth Legion as a legion.

Horus was NOT happy in the battle's aftermath. Far from gloating, he was sour over the considerable losses his own forces had incurred, but more than that, he was rattled that Russ had bested him. Horus hadn't considered Russ his equal even before he fell to Chaos, yet now, even being infused with additional power from the Ruinous Powers, Russ had laid him low and legitimately could have killed him, had Russ' feelings not gotten the better of him. Horus spent time ruminating on this, fully acknowledging it to himself. It was amidst these meditations that Horus then suddenly began to experience moments of visions, or perhaps glimpses of visions of his conscience, or something to that effect, berating him for betraying his father, becoming a pawn of the Ruinous Powers, and ultimately just doing what he knew to be wrong. It was this last that most unsettled Horus as this had never happened before, and try as he might, he couldn't simply dismiss it; unbeknownst to him at the time, the wound Russ had dealt him had impacted his spirit as much as his body, and the power of the Emperor's light had slowly started to counteract the baleful malaise of the chaos gods. Flustered and annoyed, he then ordered a certain armless failure to lead a contingent of his forces to hunt down the remnants of the VIth and not return without Russ' head. Disappointmaddon then returned without Russ' head. Oh, and without most of the astartes Horus had given him as well. Abaddon basically starting as he meant to go on.

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, granting him enough self control to reject the Chaos Gods but continued the campaign on his own terms. Lorgar believed that Horus' failure to submit to the gods' will but still accepting their gifts would lead them to withdraw their power when he needed it most and cause the war to be lost. Horus later fell into a coma at the aforementioned battle at Beta-Garmon as his wound reopened, and Lorgar attempted a coup.

A shard of Horus' soul that had been purified by the Spear of Russ would be fought over by the Chaos Gods, requiring the sacrifice of Maloghurst in order to free him and Lorgar was expelled from the Warmaster's fleet after his coup failed, with Horus warning that he would kill Lorgar if he ever saw him again.