Author's Note: To clarify some questions that might come up later, I've decided to write about a few of the directions I took whilst writing this story. This story is set before any of the events in the World of Warcraft universe. I'm old. I played the RTS games and not the MMO. As such, the story I am familiar with is that of Archimonde dying to the wisps of the World Tree as he ascended Mount Hyjal. The events of this story would take place a year (or was it two years) after this. For the Warhammer 40k side of things, I've decided to set the events during the 43rd Millennium with the Imperium on the brink of collapse. Any terms readers might not be familiar with are included in the notes at the end. The date format for the Warcraft universe is also explained in the end notes. Please note as well that I am in no way an expert in any of the universes. I played Warcraft as an RTS and read a few books on it. The same goes for the Warhammer 40k universe. I derive my knowledge from the books I've read and the various wiki's available to me. Forgive me if I breach any canonical claims. This is, after all, fan fiction. Also, English is not my first language, so pardon any grammatical errors I may have. Enjoy!
On Grey Knights: The Grey Knights Chapter is a carefully guarded secret. They are the military arm of the Ordo Malleus (the Daemon Hunters of the Inquisition) and are deployed whenever great Daemonic excursions occur throughout the Imperium.
Disclaimer: The universe of Warcraft belongs to Blizzard while anything from Warhammer 40k belongs to Games Workshop. Grey Knight name and plot points belong to Ben Counter and the Black Library. I own nothing save the cross-over plot.

"The Eye of Terror is a realm of madness and despair, where skies weep blood, ancient stars burn in multi-coloured flares and the whims of the Dark Gods of Chaos hold sway. Synonymous with discord, terror, ancient secrets and insanity, no other place in the galaxy fills the mind with such dread and foreboding."

— Extract from the Galaxia Daemonica Perpetua of Jerome (By order of Inquisitor Dalma, 453.M35)

The Maelstrom, a chaotic rift that marked the Great Sundering of Kalimdor, was stirring. Centuries had passed since its conception and now, after the destruction of Nordrassil, centuries of relative stability succumb to chaos. The world would know no peace; no reprieve from the horrors that are fated to come. Brought about by a sudden storm, a gateway to another world is opening. The Maelstrom writhes, as if reality itself was about to be torn asunder. Time is erratic; the storm existing one moment and vanishing the next. It is finally beginning – a season of fire and blood. Look to the clouds and behold the eye.

302.M42, Segmentum Obscurus, Cadian Sector – aboard the Grey Knight Strike-Cruiser Invictus

At first, no one believed them. The inquisitorial fleet must have been mistaken claimed many of the High Lords of Terra, going so far as to be called into question by many of the Adeptus Astartes chapters that loathed the organization's work ethic. Many would not accept that Cadia stood once more and that, once again, it stood to change the fate of the galaxy. It was a schism in the hierarchy that the Imperium of Man could ill afford, especially now in the 43rd Millennium, when the Emperor was at his weakest. Slowly but surely, the Imperium had devolved into a galaxy wide power struggle. Now that the end seemed almost inevitable, splinter groups had started forming, claiming their way of life would save humanity. Whole systems had fallen to a variety of xenos forces, while others were struck down by their fellow men. Worse still were the systems claimed by the expanding influence of Chaos. Humanity was finally at the brink, and even the mighty angels of the Emperor were no longer enough.

Yet, there were still those who'd kept vigil throughout the long night – those who remained steadfast in their faith to the God-Emperor of Mankind. For what else were they made for than to combat the vilest of the Imperium's foes. Indeed, the countless fleets of xenos paled in comparison to the daemonic entities of the Warp. They were the militant arm of the Ordo Malleus – the daemon hunters of the Inquisition. They were meant to combat the Ruinous Powers by sword, shield, and faith. They were the Grey Knights.

It was the reveal of their existence by the Ordo Malleus that spurred at least some Adeptus Astartes chapters to see sense and act. Chief among those that rose to the call of battle were the sons of Rogal Dorn, the Imperial Fists and their brother chapter, the Black Templars. A mustered Imperial fleet had then left Segmentum Solar. It was smaller than the first battlefleet that had charted its course to defend Cadia during the 13th Black Crusade. However, there was nothing to be done. The inquisitorial fleet that had first brought the news claimed that Cadia was whole once more. Whether it be through the might of one or all four of the Chaos gods was inconsequential. What mattered was a new warp rift was being called to being by the denizens of New Cadia. One so large that it was predicted to rival the size of the present Eye of Terror. It was the new Cadian Gate – the foothold the Ruinous Powers needed to maintain a permanent presence in the Materium. If allowed to expand, the Imperium would fall to its knees. The wanton genocide that would follow was surely a given.

Thus, it was through these series of events that Grand Master Mandulis of the Grey Knights 1st Brotherhood, found himself and his ship battling Chaos fleets in Cadia's high orbit. It was testament to the chapter's might that the ship still held together since they'd exited the Warp only hours ago. They were prepared for the worst and still they were caught unawares by the sheer number of Chaos ships the enemy's battlefleet had fielded. They were as innumerable as the stars – a blockade that would be more than a match for the whole Imperium's fleets combined. The paltry sum of Imperial ships that rode to meet them was laughable in comparison. They called this endeavor the Crusade of Terror; an apt title by Mandulis' reckoning. Indeed, it was terrifying to face such numbers with the amount of ships they had. The hilarity of the situation was not lost to the Grey Knight. This was their duty, even in the face of such impossible odds.

"Grand Master, void shields are almost down. The engines are compensating for the extended evasive maneuvers but expect the reactor to flat-line if we keep at this pace for more than 10 minutes."

"Shunt coolant into the reactors and push them to 120%. Re-route auxiliary power to maintaining the void shields and the engines. Our primary goal is to have our Knights on the surface of Cadia. We must achieve planetfall."

"Aye, Grand Master. The Emperor's will be done."

Mandulis stared at the Invictus' pict-screens once more. They were making steady progress. The debris field that formed did little to hinder their approach to Cadia. His biggest problem now was to try breaking through the Chaos blockade that seemed to loom across his vision. They were severely outgunned. It was only through the efforts of their escort ships, mostly through taking the brunt of the enemy's cannon fire, that the Invictus had made it this far. They needed something daring.

"Captain, prepare the ship for collision. Ram that battle-barge ahead. Plot a course to ram and deflect. Hopefully, we manage to break through their lines and deploy at high atmosphere."

"Plotting course now, Grand Master. Hailing escort ships to form up around our strike-cruiser."

Mandulis grinned. It was finally the Grey Knights shining hour. They had been bred for this. Raised to embody the Emperor and carry out his will. Now that the fate of humanity hung in the balance, the Grey Knights were there to tip the scales. This was his purpose. And though none may survive to remember his and his brothers' deeds, the Grey Knights died easy knowing that His will had been done. With that empowering thought, Grand Master Mandulis reached for the ship-wide vox.

"All hands brace for collision. Remaining Grey Knight detachments must prepare for immediate drop once we break through the blockade. This is our moment, brothers. We are his hammer and the bane of daemons. Let us die together on Cadia's surface."

The ship-wide transmission cut out. Mandulis strode towards the captain's chair and braced himself for the oncoming collision – the looming façade of a Chaos battle-barge filling the pict-screens.

"BRACE!"

Invictus met the Chaos-tainted ship head on. The nose of the strike-cruiser struck first as the void shields finally overloaded and failed. There was a mighty groan as the metal met metal in a mighty crash. For a split second, it almost seemed as if the Chaos ship would absorb the blow – as if the Invictus would fold in on itself rather than deflecting to the side. Thankfully, the escort ships collided not a second too late. The Armageddon, a Black Templar cruiser managed to impale itself into the Chaos battle-barge, jutting out like a javelin that struck prey. An explosion soon followed and the way to the planet was suddenly clear. Invictus relinquished its efforts into re-powering the void shields in favor of pushing the engine to maximum capacity. The Grey Knight strike-cruiser rocketed forward despite ships from the blockade turning to pepper their exposed hull with ordinance.

Throughout it all, Mandulis looked proud of the bridge officers. They faced the challenges head on without fear of death. Such was the character of all Grey Knight vessel personnel. They'd been conditioned to know no fear and to do their duty at the cost of their life. It was a marvel to behold such dedication to the Emperor's work. Although they were automatons and very far from being human, Mandulis was proud to have served beside them even if they weren't Grey Knights.

"Captain, I will prepare myself for the drop. Get us as close to the planet as you can and signal the drop. The Emperor protects."

"The Emperor protects, Grand Master."

With that, Grand Master Mandulis of the 1st Grey Knight Brotherhood departed the bridge. There were prayers and preparations to be said and made. He was burning in anticipation to once again banish daemons back to the Warp. Little did he know that there was no glory to be found in the Cadian system; that he would not find a glorious death on the field of battle. No, Grand Master Mandulis would die a most ignoble death. He would die in high orbit. Long before he could see an entity of the Warp.

Brother-Captain Alaric surveyed his squad. They had been on stand-by for what felt like hours, each keeping vigil as they recited last minute purifying rites and prayers. Everyone in the holding area was aware of the battle raging across the void. The Grand Master's vox transmission served merely to remind them that they were about to enter the fray themselves. In any other situation, Alaric would have been wary of who he was entering battle with, for even the Adeptus Astartes were known to succumb to the lures of Chaos. However, they were not merely Astartes. They were of the Emperor himself. They were Grey Knights – daemon slayers and humanity's bulwark against the tides of Chaos.

The youngest of the lot was Tancred, newly appointed to his squad of ten. His namesake was a former comrade of Alaric – Justicar Tancred who stood with him to face the Daemon Prince Ghargatuloth in the 41st millennium. His ex-comrade had given his life so that Alaric could banish Tzeentch's champion back to the warp. The neophyte had a lot to do before he could prove himself worthy of the name. To Alaric's right was Dvorn. He was unique among the squad due to massive force hammer he wielded. It was rare for a Grey Knight terminator to wield one, and Dvorn prided himself in having mastered the art of its deadly use. Karlin stood beside Dvorn – one of the members of the original Squad Tancred. Karlin was clad, much like Dvorn, in a suit of Terminator armor – a suit that made both figures walking tanks on the battlefield. Karlin wielded his preferred Nemesis halberd, and made up the duo of Terminators that Alaric's squad relied on during heavy operations. The other members of the squad were Justicar Genhain, Tharn, Ondurin, Lykkos, Haulvaurn, and Salkin. All were veterans during the battle for the Trail of St. Evisser, the final three even belonging to Alaric's original squad when he was still a Justicar. They all played a part in the banishing of the Prince of a Thousand Faces three centuries before, as well as countless crusades throughout the centuries since. Alaric would not have any others by his side and trusted them with his life.

"We are nearing Cadia. All hands prepare to drop," cackled the captain's voice through the ship-wide vox.

Squad Alaric moved as one. They'd been through this many times before. There never was time for reprieve in a Grey Knight's life. Their nemesis never rested thus their vigil is eternal. Their duty was to rid the galaxy of daemons, of which there was no shortage. The stakes today, however, were higher. If they failed here, they could only hope that the other half of the chapter, left to defend Holy Terra, would succeed in protecting the Imperium. This Crusade of Terror would determine the fate of humanity.

There was a dull roar. Something had happened to the Invictus, Alaric was sure of it. Without the usual drop warnings, the standard drop pod Squad Alaric occupied was shot out of the ship. It was a premature drop as the altitude readings had not yet indicated that the ship had achieved optimal drop altitudes. As the pod left the Invictus, Alaric caught a glimpse of the ship through the small viewport. It had been shorn in half, fire blossoming along its length as the Despoiler's ships laid waste to the shield-less cruiser. His vision was torn away from the spectacle as the pod spun erratically in high orbit. He could not see anything as the drop pod tumbled chaotically – the maneuvering thrusters must have also been hit. In a situation like this, all the Brother-Captain could do was pray. Raising his voice over the vox, Alaric joined his men in their devotion.

"I am the Hammer;" a lurch and the pod shifted upright.

"I am the Sword;" streaks of fire could be seen as the pod's thrusters partially activated, propelling them downward.

"I am the Spear;" Tancred adjusted his grip on his Nemesis Halberd as the pod was tugged slightly upward.

"I am the Shield;" the pod was decelerating; there was no doubt about it. Something was pulling them back up.

"I am the soldier at the End of Time;" the pod was accelerating upwards, towards the gaping daemonic warp rift that had opened in Cadia's orbit. They had been dropped too high in the atmosphere to escape its pull.

"Ave Imperator! We who are about to die, salute you."

Before anyone could react, the Warp rift swallowed the drop pod whole.

"Only in death does duty end;" squad Alaric knew only darkness.

Year 25, Somewhere over The Great Sea, Azeroth.

Cydd Shortbit was an eccentric gnome. Despite the fact that one could argue that all gnomes were, in fact, eccentric, Cydd was different from the rest. For most of his life, the gnome had dedicated himself to perfecting the science of flight. He would not settle for anything else. Thus, in the year 25, amidst the chaos that plagued Azeroth, Cydd was leisurely flying. He was flying his very new prototype zeppelin. He called it the very first, truly gnomish zeppelin. Cydd was proud of his work. His contemporaries, on the other hand, called it the "ugliest thing to grace the skies." Cydd Shortbit could have cared less. It did not matter to the tinkerer that the craft looked horrendous; indeed, he wasn't inclined to dissuade the notion. What he did care about was the speed in which it flew and he had boasted of it to the officials down in Stormwind. As ugly as it was, the craft was fast enough that all you would see was a blur. Of course, it was exaggerated, but he needed the funding to finally build it; it was schematic no longer. Upon its completion, the officials of Stormwind had put him to a test – a diplomatic mission to the late Jaina Proudmoore's city of Theramore. The king had been kidnapped and the Alliance needed to make allies and not enemies. What better way to test out the zeppelin's speed than crossing the Great Sea on a diplomatic mission. Cydd had hurriedly accepted.

However, journeys were never without their surprises and the one presently unfolding before his eyes was testament to that. Looking out the viewport, Cydd observed that the ever-stormy region of the Maelstrom was larger than usual. Pilots like him usually navigated closer to the storm due to the strong winds that would propel his craft faster (a risk he took to ensure that he made it back to Stormwind in record time). The usual lane he took was different today with the fact that it now stood in the path of the storm. The storm had previously never extended this far. Something was off. Contrary to Cydd's dedication to the perfection of his craft, the unusual storm sparked his interest. Something was drawing him closer. It was as if the Maelstrom called to him; akin to the voice at the back of one's head. Come. Fueled by the curiosity his gnomish ancestors were known for as well as the summons of the strange voice, the pilot steered the zeppelin towards the eye of the storm; the center of the Maelstrom.

The winds were strong, of that he was sure, as it was getting hard to keep zeppelin going in a straight line. He'd surmised that he had to ride out the currents if he wanted to closer to the center. As he fought for control of the steering wheel, a particular gust of wind knocked the nose of the craft off course. Before Cydd could perform a course correction, a flash of blinding red light filled his vision. It was like staring at an explosion, the image of fire burned on the gnome's retinas. As he struggled to rub away the blindness, Cydd noticed that the zeppelin had stopped moving. It was as if the world itself had stopped. Yet, for some freak reason, he could still move. Looking outside, Cydd confirmed that everything had indeed stopped save for himself. The scene that greeted him was ominous.

A pillar of red light started from the very center of the Maelstrom's vortex. It stretched from the sea all the way into the sky, as if piercing the heavens. Indeed, it was as if a great wound had been opened in the sky, parting the clouds along its length. It was a massive tear that when looked at directly, made no sense to the mind. Cydd could not fathom what he was seeing. It was neither dark nor light; neither upwards nor downwards; neither something nor nothing. It made sense and it didn't. Little did he know that this was chaos in its purest form. Staring at it for too long would drive even the most steadfast mind to madness. Cydd, thankfully, directed his attention elsewhere. Surrounding the tear was a foreboding sky that had turned black as night. There were no stars and it seemed as if all light had been snuffed out. The only sources of illumination were the streaks of lightning that were frozen as they grasped at the open sea below, and the red pillar of light that stabbed upward. Cydd was still trying to process the sight when time seemed to resume. The wheel of the craft spun out of control as the pilot was once again struggling to steady the zeppelin. He was spitting out profanities that would make even his mother blush when he suddenly spotted something – no there were two of them – came streaking out of the rent in the sky. They were like shooting stars.

He was about to turn the craft around when suddenly he felt a chill run up his spine. It was at this point that future scholarly skeptics would argue against the validity of the tale of Cydd Shortbit, however, upon seeing the zeppelin's wreck, had all arrived at a general consensus: that Cydd Shortbit lived a horrifically short life. For at the very moment when he decided to turn back, something prevented him from doing so. This intrusion came from a massive hand that encircled his neck whilst lifting him up. The gnome could not see his attacker but he could sense the wrongness that seemed to radiate from the creature behind him. A fell language sounded across the zeppelin's deck. It sounded as if it were coming from every direction. The creature was chanting. Cydd would be damned if he went down without a fight and planned to do something once the grip loosened.

It never did. The next moment, Cydd Shortbit drew his last breath as the daemon snapped his neck.

Glossary:
(1) Nordrassil - This is the World Tree in Darnassian (the dialect of the Night Elves).
(2) Grand Master - The Grey Knights Chapter is organized in this descending manner: Supreme Grand Master, Grand Master, Paladin, Brother-Captain, Justicar, Grey Knight Brother. Every Grand Master is a leader of one of the major Brotherhoods within the Chapter (each Brotherhood is comprised of 100 marines). A Paladin, usually in a squad, is the bodyguard of the Grand Master. A Brother-Captain takes command of the 100 Battle-Brothers in a Brotherhood at the forefront of the battlefield, only lower in standing to the Brotherhood's Grand Master. A Justicar is a squad commander.
(3) Ordo Malleus - the Ordo Malleus is the "daemon hunting" arm of the Inquisition. They are in charge of finding and quelling daemonic incursions within the Imperium of Man with their military might manifested in the Grey Knights Chapter.
(4) Despoiler - Abaddon the Despoiler was the second-in-command of Horus and is a champion of all four gods of Chaos. For those unfamiliar with who Horus is, just know that he was the OG Traitor to the Imperium of Man; his actions sped up the inevitable doom of humanity.
(5) Materium - It is the other part of the duo "Materium and Immaterium." Think of the Materium as where we are; 4d space where we exist. The Immaterium is where the gods of Chaos reside. In order for a Daemon to run rampant in the Materium, they must be first summoned into physical being (thus, cults exists to bring these daemons from the immaterium to the materium).
(7) Ghargatuloth - The Prince of Thousand Faces; he is a lieutenant of the Chaos god of Change, Tzeentch. He is a fearsome foe that needed be banished twice. The first time, it took 300 Grey Knights to banish him back to the Warp. Of the 300, only 1 body was recovered (he was also dead, rip).
(8) Tzeentch - The Chaos god of Change. Cannot be perceived for he is ever changing. Think the realm of Cthulhu in that we can't realize him/her because we lack the capacity to; he is ever changing. Heck, even Ghargatuloth couldn't make sense of Tzeentch in the books.
(9) Trail of St. Evisser - This is a sector in the galaxy where, in the year 999.M41, Ghargatuloth fell for a second time. Brother-Captain Alaric (who was then only a Justicar) and the deceased Inquisitor Ligeia led three squads to prevent the daemon prince's summoning. The daemon was banished but at great cost.
(10) Ave Imperator! - This is in Latin because for some reason (pardon the ignorance if ever), the Imperium of Man makes use of what they call High Gothic (or Latin). This phrase means "Hail Emperor."
(11) Year 25 - As I said, I am not familiar with the date system of Warcraft. I decided to use the wiki's timeline in that this year is in relation to the end of the First War.