This is my first crossover story; English is not my first language so forgive me if you see spelling errors.
I have also limited knowledge of all the lore for Warhammer 40k beyond surface level and all I can find investigating, so please tell me if you see any inconsistencies and I will correct them later.
Please enjoy.
Prologue
It is the 91st Millennium.
For more than six hundred centuries The Emperor has sat immobile on the Golden Throne of Earth.
He is the Master of Mankind by the will of the gods, and master of a million worlds by the might of his inexhaustible armies.
He is a rotting carcass writhing invisibly with power from the Dark Age of Technology.
He is the Carrion Lord of the Imperium for whom a thousand souls are sacrificed every day, so that he may never truly die.
But in this is about to change, for the enemies of mankind are at knocking at the doors of the Palace of Terra and the End of Days has arrived.
Only the forces of the Imperium stand against them, and the war rages for the fate of the Galaxy.
An alliance between races has been formed to stop the dark future that awaits the universe, and the planet is tinted red with the blood shed in the conflict.
This is the final tale of those times.
"They are almost here!" Yelled Trajann Valoris above the roar of monsters at the other side of the gates," None shall pass!".
The man was plated in Golden armour and wielded an ornated axe named the Watcher's Axe, he was surrounded by fellow warriors in similar armour named Custodes.
All of them were inside the Palace of Terra defending the throne chamber. The size of it easily dwarfing most skyscrapers, adorned in gold and imaginery of battles won.
The Throne was situated at the end of a number of stairs that almost reached the ceiling and the corpse of the God-Emperor of Mankind rested there. It was a skeleton seated in a Throne of gold and technology, and He was connected to It by wires and other devices.
"The foul daemons have breached the Ultimate Wall!" said back one of the men," None shall pass!".
"Steady men", Trajann exhaled air to temper his pulse, then a terrible thunder was heard like a giant was felled. "For the Emperor!"
"For the Emperor!" Screamed all the men in the chamber.
XXX
Outside of the palace, in the streets the slaughter and fighting of warriors made the Palace feel like a flood of bodies had reached it.
Men, monsters, and physical gods battled in the planet. The forces of the Imperium lead by the Primarch Roboute Guilliman fought to defend the Palace from the assault of a horde of daemons while followed by the Living Saint Celestine.
The Primarch was an imposing figure of greater size tan a human wreathed in blue armour adorned with gold and white, iconography similar to Roman culture and armed with a sword wreathed in flames used previously by the Emperor himself, that he swinged left and right decapitating daemons and enemy warriors.
Celestine was a woman in golden armour and an halo that made one remember the stories and paintings of angels. She also wielded a sword called the Ardent blade that destroyed her enemies without mercy.
If one were to be guess the outcome of the battle, he would fail to determine a clear victor of the conflict.
"Great Archmagos Belisarius!" Roboute said to his companion in red." We must destroy these foul creatures if we hope to gain ground! The enemies have us surrounded. Where are your forces?!"
The thing that was once a man had so many alterations to his body that it will be not too far to call him a robot. The cyborg wielded a polearm with an axe in the form of a cog and had his body modified with a series of mechanical parts in accordance with the creed of his people the Adeptus Mechanicus.
"+++Mechanicus Forces remaining in the Palace: 20%+++ Possibility of defeat of the Imperium: 500%, Suggestion: Retreat to the Sanctum Imperialis and join in the defence of the Throne."
"Lord Guilliman!" An Ultramarine walked backwards while shooting the guns attached to his hands to his Primarch side." We will give you a window! Please leave this fighting to us!"
"Thank you, Chapter Master Marneus!" The Primarch noded, "Lady Celestine!"
The woman flew to the man side while shooting holy rays of power that incinerated four daemons in an instant.
The three (the Primarch, the Saint and the Archmagos), then marched to the Palace, hoping it was not too late.
XXX
Not only the war was being fought between the Imperium and the daemons, but other players were also on the planet, some to aid the Imperium in temporary alliances and others to use this opportunity to gain its own objectives.
The allied forces consisted of the Aeldari, a race of people with an aspect that would make one think of elves of popular culture, leaded by Yvraine the priestess chosen by their god of death Ynnead, the Necrons, a race of technologically advanced mechanical skeletons leaded by the Silent King Szarekh and the Farsight Enclaves ruled by the eponymous Farsight of the T'au race.
The factions that made it to the planet for their own reasons were the Orks a race of green skinned bipeds that loved to fight other races and among themselves whose leader was an armoured warrior named Ghazhkull and the Tyranids a race of insectoid hive minded monsters leaded by the Swarmlord.
The war was heating up to a boiling point, and with no sign of ending without the destruction of all its participants but this was about to change…
XXX
Be'lakor a Daemon of Chaos of skin black like coal and wings the span of a room, wielding a sword the size of a grown man stared at the Eternity Gate, at his sides an horde of daemons awaited his orders.
Mere minutes before he had felled the guardians of the Gate, and knew the moment the gates opened, the psychic power of the Anathema would kill anyone weaker than a Daemon Prince.
The human traitors that had pledged their allegiance to Chaos were fighting their fellow men outside and the final conflict was approaching.
"The Corpse-God is guarded by his most trusted servants, but the victory of Chaos is assured." Thought the Daemon. The he said" It's time to end this once and for all!"
XXX
The doors opened and Trajann steadied his weapon.
At once an horde of abominations of all shapes and sizes poured in the room but the Throne projected a wave of Golden light that burned them to a crisp.
The only one that remained standing was a monstrous creature that wielded a sword called the Blade of Shadows.
"Charge!" Trajann did not await to see if his order was carried out and ran towards the foul daemon with his axe high in the air.
"AHHHH!" His fellow Custodes yelled back.
XXX
Yvraine fought daemons left and right, all of them wanting to kill her without hesitation, and then the enemies paused and parted to make way to one of them.
"You!" Yvraine eyes widened.
"Hello again Yvraine" The daemon smiled.
This creature was Shalaxi Helbane, a daemon with four arms wielding a spear and a shield that had pledged its allegiance to Slaanesh
"This time I will end you" Yvraine glared.
"Oh, don't make such a face!" Shalaxi taunted," We have come to take you to Her"
"She-Who-Thirst will die by Ynnead's hand when we take back the last cronesword! I will never join you"
"But my lord wants you Yvraine" This time the daemon looked at her seriously," You will be His bride for It has so many sights to show you!"
XXX
The moment the Primarch saw and reached the Eternity Gate he started to increase the speed of his steps and raised the sword in his hand.
Entering the room, he saw the bodies of daemons and Custodes on the floor and raised his head to look directly to the Throne were his father the Emperor rested.
Trajann Valoris was wounded and tried to stand up at the foot of the stairs.
"NO!" Yelled Roboute at the top of his lungs.
The creature turned his head in their direction and narrowed his eyes in fury.
At once, he raised his hand with the sword once more and made it descend in the direction of the Anathema.
And then an explosion of golden light blinded them.
XXX
Ghazhkull Mag Uruk Thraka shot his four barrelled cannon at the Adeptus Astartes of the Space Wolves and killed three of them at once.
"OI YA GITZS!" he screamed to his men and pointed with his clawed arm at the Palace" LETZ HA A PROPA KRUMPIMG WIT TE UMIES A IT PANZI OUSE!"
"WAAAAAAAAGH!" The orks that were with him screamed back.
But then a terrible explosion happened. A golden light blinded all the combatants and Ghazhkull lost consciousness.
XXX
The forces of the Death Guard lead by the Traitor Primarch Mortarion fought the Imperial Fist on one side of the planet far away from the Palace.
Their armour was filled with sickness and rot and their Primarch wielded a scythe and toxic fumes that made their enemies agonize and suffer.
It was then that the battleground was bathed in light and Mortarion lost sight of the world.
XXX
The Swarmlord fought T'au warriors and mechs for days, wielding four swords in a fury of Blades that brought death to his enemies in an instant.
It was at the moment when he clashed Blades with the T'au warrior known as Farsight that the world was consumed by light and his connexion with the Hive Mind was lost.
XXX
Be'lakor trike at the Emperor bathed the entire planet in light that rivalled a star and opened a gate in space and time.
The entire solar system collapsed on itself and destroyed all caught in the blast.
In the galaxy itself the psychic races suffered seizures and died, astropaths of the Imperium cried tears of blood and the Great Rift that connected the material universe with the Inmaterium extended outside the galaxy.
It was the end of the conflict and the Imperium had lost.