Re:Interdimensional Mercenary

While waiting for the inevitable Horus Heresy, the Emperor had compiled a complete chronological list of all major events up to the very end of the 41th Millennium.

Within that list is the First Black Crusade, the attempt of Abaddon to declare himself the heir of Horus and band together the Forces of Chaos and the Traitor Forces that fell during the betrayal of Horus.

Adam had been spending eight centuries, planning and preparing for this assault in secrecy. Known only to the most trusted of the Custodies that currently serve him as he Emperor willed.

The preparation ranged from secret stockpiles of arms, munitions and war gear all across the Cadian Sectors in the Segmentum Obscurus and also fortifying the worlds close to the Eye of Terror.

Even though it is impossible to change the outcome of the predetermined results, he could still subtly change some outcome like lesser losses and dealing more damage to the Forces of Chaos.

But perhaps his greatest regret is his failure to preserve even one loyal Primarch to the Imperium, leaving it vulnerable to the increasingly rampant Demonic Corruption and corruption within its leadership itself. It is simply impossible for Adam alone to lead the whole Imperium and guide it into the right path without trust worthy vassals. His influence couldn't spread far enough to cover the entire Imperium. Perhaps, that is also a predestined outcome.

Once the Day Forces of Chaos marched out the Eye of Terror, the First Black Crusade was approaching. Adam left the Imperium in the hand of his carefully picked and raised Council of High Lords while he boarded the fortress-monastery that he ordered the Fabricator of Mars to create for him.

Alongside him is the Legion of Grey Knights. He had never intended to follow the Codex Astartes that Guilliman passed to limit the size of a Space Marine Chapters which is done out of fear a leader may be corrupted by evil and bring forth destruction to the Imperium.

Adam is different. Not only is he immune to the Corruption of Chaos, his Grey Knights are too. Created from the Gene-Seed of his own which was originally the Gene-Seed of the Emperor, perfected using his body and created to be the Bane of Daemon and formidable Psyker just like the Emperor is. On top of that, they have a unique Pariah Gene Mutation that the Emperor created which although not as effective as the original, the new Grey Knights also gained massive resistance to Psychic power, a downgrade of his Psychic Immunity.

With all these preparations, Adam led his Legion of Grey Knights to the battlefield, ready to unleash his wrath upon Abaddon and his Legions.

———

"My Lord, the Grand Masters requires your presence. We had arrived at the Cadian Sectors and the Forces of Chaos had indeed encroached upon the real-space as Milord predicted."

Adam awakened from his meditation and saw his loyal attendant, the same woman that Malcador had assigned to him. Fashioned downgraded Aegis Armor to her use since she lacks the Genetic Implant necessary to wear a proper Aegis Armor. With a white robe covering her body bearing the Chapter's badge hiding the two Nemesis Force Falchions underneath.

"Give me a moment, Alina." Adam stood up and manipulated with his Psychic Power to don his larger Aegis Terminator Armor because unlike his Grey Knights, his size is nearly that of a Primarch, standing almost a meter taller than his Astartes.

After his Armor is secured to his body, he carries his weapon of choice, Titansword, a relic weapon of the Grey Knights and then he leaves his meditation room along with Alina trailing behind him like his own shadow while they head for the Command Room.

Upon his arrival, silence descended upon the room as the only noise came from the communication devices, broadcasting the distress calls from numerous worlds and Imperial forces that were caught off guard by the sudden attack from the Forces of Chaos.

"Your order, Milord?" The Grand Masters all waited in silence yet underneath their calm facades are cold seething rage and determination.

Each of the Grand Masters had been with him since the establishment of the Grey Knights Chapter, they were there with him as he waged his long grueling war in the Immaterium. They all shared the worry of Adam who they see as no different than their Gene Father. They share his disappointment in themselves for their failure to prevent Horus' treachery and now there is a wannabe that declared himself Horus' heir? Their blood is boiling to rip Abaddon into pieces along with the Traitors and Daemons that is his Legion.

"Ignore the distress calls, our destination is the Immaterium. Once we cut off their reinforcement, the Abaddon's crusade would definitely be cut short." Adam said and reached for his seat.

"As you willed."

"Set course to the Eye of Terror, full speed ahead."

"Preparing for Warp-Drive Jump."

As such, the fortress-monastery plunged itself into the massive Warp rift. Confusing the Chaos Fleet that are busy invading the Cadian Sectors.

———

Abaddon, the ambitious self proclaimed heir of Horus. Was busy waging war on the real-space totally oblivious of the intruder slipping into the Eye of Chaos. That is until his Patron Dark Gods start to make a big fuss.

The Grey Knights Chapter had faded away from the surface after the Horus Heresy. It didn't help that even before that, the Chapter's existence is known only to few as the battlefield they frequent are those that are always infested with Demonic Hordes and none are willing to fight on because of the immense difficulty to recapture and often candidates of Exterminatus.

The Grey Knights are not even known to most Primarch, much less a mere Astartes that Abaddon were prior to him fallen into Chaos and neither does his patrons divulged the existence of the Grey Knights to their chosen Champions because they thought the Grey Knights had died in the Necron Tomb World that day. Never expecting them to not only survive but also grow in greater force and might.

By the time Abaddon was called upon by the Chaos Gods, the Grey Knights already lay siege to the heart of the Realm of Chaos, causing rampant destruction and devastation in hope to cripple the Forces of Chaos off their logistics and reinforcement.

This realization caused Abaddon to be furious. After all, the advantage of Chaos Forces is their inability to truly die and will descend upon the reality with great numbers to overwhelm the defenders of the Imperium. But the Demon was killed with true death, not weakened and forced to recuperate their power before descending into reality once more. This shatters his plan to overwhelm the still recovering Imperium from the scar of Horus' betrayal.

With his Legion cut off its reinforcement, the swiftly arriving Imperial Guard Regiments, Imperial Navy and Astartes Chapters pushed back Abaddon's Legion into the Eye of Terror. Marking its swift end with only two dozen Terran Years spent on this otherwise long and bitter grueling war.

From then onward, each subsequent Black Crusade was thwarted with overwhelming victory. Because in the previous lifetime of the Emperor, the reason why each subsequent Black Crusade is more devastating than the previous is due to the advantage of the Chaos Forces to suffer no true death and their numbers continue to swell greater in number as the Black Crusade continues while the Imperium suffer more and more devastating losses despite their victory.

What Adam did was simply evening the playing field. If the Imperium suffers that much damage, the Chaos must too.

It went to the extent that all four Chaos Gods tried to assassinate him but to no avail because his mortal body is immortal, a Perpetual. While his Soul is immune to the Psychic power of the Immaterium. Killing him is as effective as hitting the water to kill the ocean.

But, the Chaos Gods and Abaddon are not fools. Knowing their methods are not effective, they adapt and change.

The 9th Black Crusade, the battle where the Imperium suffers the greatest wound.

Knowing themselves to be insufficient to overcome Adam and his Legion of Grey Knights due to their specialty in combating the Daemon and Forces of Chaos. Abaddon works together with his Patrons to lure other Xenos races into the fray, battering the Imperium with numerous attacks from Necrons, Orks and Tyranids. And it worked.

Adam and his Grey Knights Legion suffer devastating losses when the Chaos Gods lured him into a battle against a Tyranids Hive Fleet after banishing him and his legion into the far corner of the galaxy.

The unprepared Grey Knights fought tooth and nail, cut off from their supplies while equipped with inadequate weaponry to fight against the Hive Fleet. It was a pyrrhic victory with the Grey Knights suffering major losses again and again fighting numerous battles as they made their way back to the territory of the Imperium. However, the moment they returned. Abaddon had already launched the 9th Black Crusade and his absence caused the Imperium to suffer great losses to defend against the well planned assault by the Forces of Chaos.

For the following Black Crusade, Adam and the Grey Knight always suffer in the machinations of the Chaos Gods, forcing them to be absent throughout the Black Crusades but Adam devises a method to counter them.

Adam sought out the Xenos race, striking a deal with the Aeldari to exchange protection from the Chaos God Slaanesh in exchange for safe passage through their and cooperation. Also with the Necron, secretly selling a certain slumbering C'tan on Mars for many of their technology and relics such as non Warp reliant teleportation technology, Tesseract Labyrinth and Psychic nullifying Blackstones pylons to rebuild the Cadian Pylon Formation in case Cadia still get destroyed by Abaddon.

———

At the very end of the 41th Millennium, the Terran year of 999.M41.

It was finally time. The last war before his long awaited freedom. Adam did his best in his close to ten Millennium tenure as the Imperial Regent, leading the Imperium through the bleak time after the Horus Heresy.

All sorts of preparation he had made both secretly and openly, had been set in place. His close to ten Millennium of accumulation of effort along with the Emperor for this one last war. The turning point of destiny and fate. The war that decides the survival of the Imperium.

He led an entire Battle Fleet to the space zone of the contested Fortress World Cadia, awaiting the arrival of Abaddon and his legions.

Unlike the Emperor's previous life. Throughout the Black Crusade, the Imperium had been on the passive side of the battle and could only scramble their fleets after Abaddon had already attacked. In this lifetime, the Emperor had left behind the Chronology of each attack of Abaddon. Although not always precise due to deviation of events, there's still something to guess from.

The moment Abaddon's fleet emerges from the Eye of Terror, the two grand fleets engage in a bitter fight.

As usual, the Grey Knights and their fleets charge straight into the Immaterium through the Eye of Terror.

Abaddon expected it and disregarded it because he already set a trap for the Grey Knights and Adam. Except, Adam was not inside the Fortress-monastery.

When Abaddon forces open the Warp rift on the surface of Cadia, created by the hidden cultists he is ordered to infiltrate. Nothing happened much to his confusion.

Since his plan to open a back door and attack from the inside had failed, Abaddon could only shatter the Forcefield Array covering the entire Cadia to drop his troops. But the Imperial Fleets wouldn't let him pass that easily. Fighting to the bitter end if they need to and daring Astartes Chapters and a few others like the Space Wolf board the flagship of Abaddon himself to cripple the leadership of the Chaos Forces.

For months, the Chaos Fleet and Imperial Fleet fought bitterly. But the Chaos Fleet finally managed to break through the defense line due to their superior quality of combatants, bringing their absolute best for this one last crusade while the Imperium had declined despite everything that Adam did to preserve it. He is not omnipotent like the Emperor was after all.

When the defense line was breached, the Chaos Forces finally descended upon Cadia and the years-long siege finally started.

———

Deep under the fortified cities, Kasr scattered across the surface of Cadia hides companies worth of Grey Knights along with other Chapter's Space Marines, the PDFs and Guard Regiments, reading to protect the cities.

He learned from the notes left behind by the Emperor. One of the big contributors to the defeat of the Imperials during the siege is because the PDFs and Regiments are left to themselves to protect their fortifications overruned by Traitor Astartes, Chaos Warbands and powerful Daemons that are well beyond their capability to fight against. Meanwhile the Space Marines were focusing on their own personal objectives rather than the overall objectives that is to defend the Fortress World.

This issue came mainly from lack of proper command chain and authority figures that can rally every combatants under leadership. However, with Adam here. He will exercise the authority granted to him by the Emperor and ensure each of those unruly Space Marines are properly leashed and focus on the main objective.

His arrangement is to include Space Marines in each Kasr and also Grey Knights that are effective against Daemon and Chaos aligned enemies. At least one company worth of Grey Knights defending each Kasr.

Thanks to Adam's effort to recruit as many Grey Knights as possible and have at least ten thousand of them. It's barely enough to cover each city after half of them split off and board the Fortress-monastery as a smokescreen into the Eye of Terror and at least a thousand more are assigned elsewhere to protect important assets.

When the Chaos Forces started their descent onto Cadia, they were shocked to be greeted by familiar Nemesis Force weapons. Especially the Heralds and Greater Daemons that are still traumatized by the regular excursions the Grey Knights did to cull their numbers in the Chaos Realm.

Eventually, Abaddon also learnt about the presence of the Grey Knights fighting amongst the defender of Cadia. Although he had yet to learn about the presence of Adam, think Adam had fell to the Time Dilation trap set up for him.

Forced by this unexpected development, Abaddon stops holding his punches and unleashes everything he had upon the Imperials, hoping to overwhelm them with sheer number and firepower as he too, joins the battle on the planet's surface.

For three continuous years, battle raged on across the Cadian Sectors. Deaths numbered in billions, more than a handful worlds ravaged by the war and reduced to Death Worlds. Yet, the flame of war rages further on like an unstoppable forest fire.

Somewhere around the fourth year of the war, the Aeldari Warhosts of the Craftworld Biel-Tan and their fleet arrived to reinforce the Imperial Forces to honor the pact their gods, Khaine and Cegorach, made with Adam to liberate Isha from the grip of Plague Lord, Nurgle's garden.

Not long after, Trazyn the Infinite also arrived to honor the deal he made with Adam. After all, Adam just helped him gain one of, if not, the most prized collection to him, an intact C'tan. Aiding the Imperium against the Chaos Forces is just a small matter to him when he is already going to do it anyway, much less when Trazyn regards Adam as a valuable customer and a dear 'friend'. At least now he is getting paid for the work.

With the help of unexpected allies which is only possible because Adam curbed the Anti-Xenos extremists, the Imperium pushed back the Chaos Forces into the Eye of Terror in less than a year. Resetting all the progress Abaddon had made and doomed his conquest upon reality and the Imperium.

In his last ditch effort, Abaddon noticed his 13th Crusade is doomed but unwilling to retreat without gaining anything from this war when his Patrons are already restless on their throne, driven insane by Adam's constant harassment on their realm.

So, since Adam had been sighted on the surface of Terra. Abaddon decided to sacrifice the Will of Eternity and crash it upon Cadia, destroying the world he couldn't claim with the sheer kinetic impact of the ancient mega-weapon.

With Adam. He was staring at the quickly crashing Blackstone Fortress. Order came from the Imperial Fleets above, screaming on all friendly Vox comms to evacuate the doomed planet.

Even his host of Grey Knights escorts panicked when he simply stared at the sky instead of evacuating.

"Milord…" Alina stood calmly next to him. Her armor is full of scratches as marks of enemy weapons.

Adam heaved a sigh and turned to his faithful attendant. "What is the status of our Fortress-monastery?" He asked.

"We are still unable to contact them. It seems they will not make it in time, Milord." She replied dutifully. But inevitably, her face shifts subtly to worry as his Elite Bodyguards are.

"Milord, don't tell me…" She paused when Adam lifted his hand up and stopped her and turned to the leader of his Elite Bodyguard, a venerated Grey Knights from the period of the Legion's founding and one of the most senior Grey Knights eligible to be a Grand Master yet turned it down to be a part of his personal guard.

"Order all of your Battle Brothers and all of the Imperial forces on the surface to evacuate immediately. The Imperium cannot lose any of you today. There is still more battle awaiting everyone of you in the future." Adam gave his last and final order.

Hearing the grim order from their Gene Father, the Grey hesitated but seeing the resolute face of Adam. He relent and fulfilled his order.

Then Adam turned to Alina and said,

"You too, Alina. You who had worked alongside me for almost ten millennia are an important key figure to stabilize the Imperium's politics when Roboute Guiliman took my mantle as the Imperial Regent." Adam ordered with a stern expression. Although underneath, he also felt reluctant to part ways with his closest aid throughout his life in this universe but he also didn't know he could bring her along and her usefulness to the Imperium is a far greater merit than following his selfish whim.

Of course, the moment she heard it. She resisted but Adam used his Psychic Power to put her to sleep and hand her over to his departing bodyguards as he whispered his apology.

Then Adam looked up again at the descending Blackstone Fortress. He didn't worry if Cadia fell and was destroyed. That's what the spare Black Stone Pylons are for which is installed on the Gray Knights Fortress-monastery with the help of Trazyn. The Eye of Terror will not split the Imperium apart like the Emperor mentioned and now, his final role is to buy time.

To buy time for all the friendly forces to evacuate from the surface of Cadia to survive and continue to serve the Imperium lead by Roboute Guilliman as the Emperor's true heir like the Emperor intended to while he finally gains his freedom from this universe.

———

As many evacuated from the surface of the war-ravaged Cadia, they saw a blinding white light explode and the light, noticed by many to be similar to the Astronomicon, a beacon of pure, unadulterated Psychic Power unleashed upon the physical realm.

To those not sensitive to the Psychic force, they see only a beacon of light. But to the many Psykers, they saw a giant form of a man in Aegis Terminator Armor, roaring while his hands held the Blackstone Fortress.

Those of the Grey Knights Legion are aware, the Psychic force belongs to Adam, their Gene Father and he is burning his own soul to harness a titanic amount of Psychic energy from the Warp and halt the Blackstone Fortress' descent to buy time for all friendly forces that had yet to evacuate.

As the time trickles, the Psychic beacon glows dimmer which indicates the waning of Adam's lifeforce and for five long hours, he stalls the descent of the Blackstone Fortress until his lifeforce is fully exhausted, dissipating into nothingness.

While many wept at the scene, the Chaos Gods revel with joy, knowing their greatest annoyance finally died off. But to the Grey Knights Legions, they all hardened their heart and mind that is burning in cold fury, vowing upon them everything to destroy the Forces of Chaos and defeat their Dark Gods.

—————