Roboute Guilliman is a man in torment. The Primarch of the Ultramarines woke up what seems like centuries or even millennia ago in total darkness. The only things he has ever been able to see in the darkness are his own armed and armored body and the thing that has been stalking him over the span of lifetimes that calls itself Kothrok. Kothrok, who claims to be an emissary of the Choas God of Pleasure, Slaanesh. When Kothrok wants him to anyway when, the twisted peacock daemon decides to display itself in an array of colors so twisted they hurt the eye. When it doesn't, Kothrok becomes one with the darkness. The daemon has been whispering entreaties to Roboute, trying to strip away the defenses around his will. Telling him how Slaanesh will free him from the darkness and show him universes of pleasure like he can't even imagine if he just submits. How his sons, his mighty Ultramarines, would be freed from the darkness they are all in. A darkness called the Imperium of Man.
Guilliman ignores the daemon and has always ignored it. He just walks. He has walked for generations, but never has he once spoken back to the Kothrok, never has he uttered a word. No matter how tempted he is or how badly the pain in his throat becomes, which Kothrok promises will stop if he just speaks, he never utters a word. He knows that the pain is from where Fulgrim cut his throat with his poisoned blade. He knows that this dark place is a prison of his own mind. He knows that he is sitting on a stasis throne on Macragge. He knows all of this because Kothrok has not only told him but showed him.
When the daemon tires of being ignored or when it has a particular piece of insight it wants to impart, it attacks Guilliman. When the thing's talons connect with Guilliman's power sword, the Gladius Incandor, or his power first Hand of Dominion, it shows him what is going on beyond the confines of his stasis throne and his mental prison. It showed him the rotting shadow of his father's vision the Imperium had become. It showed him his father rotting on the Golden Throne while being deified as a god by the Ecclesiarchy, which they have leveraged into a power that has buried its hooks into its rotten heart. It showed him the corruption of the High Lords of Terra. It showed him humanity drowning in blood and war. It showed him his sons still fighting for the ideals he instilled within them, only to die for an Imperium that doesn't deserve their honor. At first, he knew it was a lie; he knew it without a doubt, but the more the daemon showed him, the more that certainty crumbled.
"Why would I lie," asked Kothrok after each revelation, "When the truth is so much sweeter."
Guilliman kept his silence. He kept walking and kept battling, but he knew he was getting weaker and slower. Even his mind, so indomitable in life, is being worn down as he is denied death or rest. He was always able to stand in even ground with the daemon, but in their last half-dozen engagements, he suffered wounds. His armor is torn open, and his bleeding will not stop. He knows that the damage is not to his physical body but to his mind and maybe even his soul.
He stops walking as a pair of glowing, slitted pink eyes appear before him. Kothrok steps from the darkness. The shadows slid off it like sand, revealing the unnatural colors of its blade-like feathers. Golden chains with the still-screaming heads of long-dead hooked to them are draped over its gigantic form. Its bird-like head is complete with a red beak filled row upon row of megalodon teeth. It walks out on four horse-like legs that end in reptilian feet tipped with purple claws as long as swords.
"Oh, my," says the daemon in a sickly sweet voice filled with mock concern, "Friend Guilliman, you look absolutely exhausted. Would you like a drink? Maybe some Elyssium wine. Oh, it has been so long since you have enjoyed its taste. It has gotten so much better now, you know. A delicacy throughout the Imperium. The slaves that work the world-spanning vineyards are born into their lives. They start picking when they are four years old. Entire generations have been born and died in those fields, passing down their knowledge of the fields and the winery. If they are lucky, they won't catch the eye of the inbred nobility, who enjoy a little screaming with their wine."
Guilliman just raises his weapons as the daemon laughs and begins to circle him. He tracks it with his eyes as his legs, which feel heavy, move him in a slow circle that matches the daemon's. They both stop as a new sound feels the darkness. It sounds like someone rythemicly banging on a wall. The volume and the complexity of the best grows until there can be no doubt that someone is playing a drum in the darkness. Kothrok looks around in confusion with slowly turns to one of panic as the sound gets louder until it sounds like a thousand cannons are being used to play a symphony. Guilliman finds the melody as beautiful as he always had, and for the first time in millennia, he speaks.
"Beautiful," he says in booming baritone of a voice that has lost none of its strength, "I have never heard a more perfect rendition of the Might of Mecragge."
As if his voice was all the gunners needed to zero their target, the cannons continue to boom, and with each one, a hole is blown in the darkness, letting bright, cleansing light in. Guilliman falls to his knees and closes his eyes as the warm caresses his face.
"No," screams Kothrok as the prison of darkness is cracked open like a glass dome to the beautiful symphony of war, and they are both bathed in warm light, "No! He is mine. His mind, body, and soul were promised to me by the Prince of-"
The daemon goes silent and starts to step back in fear. When Guilliman opens his eyes, the darkness is gone. He is now kneeling in a lush green field. The sight before him takes his breath. A normal man in beautiful black and gold armor stands between Guilliman and Kothrok. Or at least he looks normal. Guilliman remembers the first time he met his true father, the Emperor of Mankind. He was in awe of the man's stature and the power of his aura. He had never felt another power like it until now. The power of the man before him is the most pure he has ever felt. He radiates it in waves, but it also flows into him as if it is fighting to be a part of him. Guilliman feels his tiredness washed away, his wounds are healed, and his armor is repaired. All just from being in the presence of this white-haired man.
"It is nice to meet you, Roboute," says the white-haired man without looking back at him, "My son made a lot of mistakes, but it would seem he succeeded with you. Or maybe you succeeded in spite of him. Take a rest, kid. Your grandfather will handle this."
"Grandfather," says Guilliman.
The weight of the implications of that one word, that one title, is heavier than a planet. Grandfather? That means that this man is claiming to be the father of his father. The Father of the Emperor of Mankind. The father of the most powerful man to have ever lived. The father of a man who was and is so powerful that an entire race worships him as a god and the sons he created as demigods. Who created legions of transhuman warriors who brought hope and fear to the galaxy. Who united humanity under the flag of the Imperium of Man. This man is claiming to be the one who gave that man life. It is an unbelievable claim on so many scales, but Guilliman believes. He believes without a single shred of doubt that he is looking upon his Grandfather.
"You can't-" starts Kothrok.
The white-haired man closes the distance so fast that even Guilliman can see it. He reappears in the air on eye level with Kothrok before driving a right hook into what passes for the daemon's jaw, sending it flying across the field. The white-haired man doesn't land. He hovers in the air. He doesn't fly after the daemon who has flown over a mile. He simply puts out his hand, and the daemon flies back to him, stopping mere inches away. One side of its face is a crater, and the massive beak hangs to one side.
"Yes, I am real," says the white-haired man, "For beings who know every, you don't know much. You aren't looking too good there, Kothrok. Let me help you out."
The white-haired man snaps his fingers, and the daemon's destroyed face instantly heals. The daemon stares at Guilliman's grandfather as wide-eyed as Guilliman does. His grandfather just smiles before dropping to the ground and firing back up to deliver an uppercut that sounds like an explosion. Kothrok just flies up and up and up…and up until it disappears from even Guillimam's sight.
His grandfather holds up a finger before shooting into the air after it. Guilliman gets to his feet as he looks up, trying to catch sight of them. There is a streak followed by an explosion that throws an earth miles into the air a few miles away, causing an earthquake. Guilliman is about to run that way when his grandfather appears in front of him, holding what is left of Kothrok, one of its eyes, in his hand.
"I swear you would think as much shit as they talk, they would be a lot tougher," he tossed the eye off to the side where it faded into smoke before it even hit the ground, "Well, it will serve nicely as a messenger to its master and the rest of its kind."
He steps forward, offering his hand to Guilliman, "I am Genesis, by the way. Let's have a chat. Then, you can decide if you want to wake up or not."