So, I was asked to give details and I am oh so happy to give them so here you go!
First off, let's start with how Roboute time traveled.
The details are a little fuzzy and I will have to work on them but I would say that the general idea is this: Roboute is killed by the High Lords of Terra. How he was killed is still up in the air but he dies. Maybe poison? Something that can carry on into the past. Put a pin in that.
How he ends up in the past is, of course, the Emperor's fault. Let's say the man yeeted him into the warp and into the past. Soul wise of course; his body, now freshly dead, stays behind.
Now, how does he react to being in the past? There are a lot of things that I can say happen. The mains ones being: being very passive about everything, not really interacting with his sons and just going through the motions.
He does not look at his sons. The faces of the long dead. He had searched for them once, in a crowd of hundreds of men wearing the same shade of blue in that same blue armor. He had, of course, come up short. The faces that stood in that crowd were his sons, that was true, but they were not sons that he knew. The faces he knew had long since turned to dust. So these faces, that stood in front him, with smiles and eyes not yet tainted by the horror of a civil war, they were not real. They were just phantoms that he had thought he put to sleep but clearly they were still awake.
Roboute did his paperwork. He drank his recafe. He signed where needed. He dotted his 'I's and crossed his 'T's. But that was all he did.
He stared at the dataslate in his hand. He turned to stare at the other dataslates that were laid out on his desk. It was endless dataslates with endless cups of recafe. His desk was covered. There was piles everywhere. He needed a bigger desk. A table maybe? Ah, yes, a table. A dinning table. One that was fit for baseline feast -- that should fit his needs. He would need an organization system to work with that kind of table. But it could work. He would make it work. He reached for another dataslate and called over his shoulder.
"Calgar," He said. "Order a table for the height of a Primarch and the length of a feast table that the Nobles own." His eyes never left the screen.
"Father," Calgar's voice was different. Altered in a way that he couldn't put his finger on. "I'm not Calgar."
Ah, that's why his voice was different. "Sorry Titus, I didn't know you have returned."
There was pause.
"I'll order the table -- anything else you need?" There was something there in that voice. Was Titus hurt? It wasn't something that was farfetched. In fact he would say it was expected. But why wasn't he with the Apothecaries?
"Rest. And get your voice looked at."
"Of course, Father."
And ignoring anything that doesn't match up with what he thought it should be.
His armor felt off. His skin around his throat felt like it was chaffing. The recafe tasted different. And his office looked like what it did before he was killed -- no, not killed, forced into a deep sleep.
It wasn't to say that the office changed in the ten thousand years. His sons froze it in time much like they froze him.
He still didn't know if he was happy or pissed that they didn't change a thing. He didn't know what would have been better. Some kind of difference or something that was an anchor in a storm.
Confusion and resignation.
That was what he settled on.
Everything he felt was normal. His normal now.
So, he's not doing to well. But he's alive? That should be worth something. Right?
Now, what is the tipping point? What is the moment that Roboute, who is barely hanging on as it is, is taken to his knees?
Well, that pin that I told you to hold? About the poison? Yeah, that comes into play.
The High Lords of Terra have tried many times to kill Roboute. They have tried in many different ways to get that job done. They have tried to set whole Chapters on him, only them to be foilled by the Ultramarines, Custodies, and sheer luck (Godly intervention). It wasn't till they poisoned his recafe store, that they managed to get him.
The poisoned clearly didn't follow Roboute into his younger body.
Roboute was sitting at his desk like he always was.
There was a time, a long ago in the time frame that the Imperium both praises and curses, that while he did spend a lot of time behind the very same desk; he could still leave the desk. The paperwork and dataslates were never in such high amounts and were easy to put to the side.
The upside of being a Primarch and not the Emperor and Malcador. Because he was not under any kind of misconception on who was doing the paper.
Roboute ran Ultramar. A thing that was so small when compared to the Imperium that the Emperor had to look after.
Roboute looked away from the dataslates and looked to the wall on the far end of the room.
His office was a pretty empty thing. It wasn't always like that. There was life in it once upon a time. Paintings made by brothers and furs laid on the floor that were hunted by other brothers. Plants that his mother cared for that were once scattered all around. She used them, he knew, as an excuse to come and check on him. She would say 'just giving them water' and then bully a dataslate out of his hand and push him out of the room when she thought he was over working himself. There used to be color. Used to be life and joy and memories that were untainted.
Now?
It's a grey mess of walls with no plants hanging on them. There was no furs on the ground or on chairs. Chairs that were no longer there. The painting were long sense removed and all that was left was empty frames.
All that was in the room was him, his desk, dataslates, a window that was just a screen in a frame, and one lone painting that he painted himself.
The painting was simple, in fact it was more like a line painting with barely in color. It was the first thing and the only thing that he had painted, back when he thought he would have the free time. Back before he realized about just how bad the Imperium truly was.
The painting was of his father, the Emperor was Mankind, in his prime.
Everyday his sons walk past it, they comment on it, they often remark that the man looks sad. Others say that there looks to be the barest hint of a smile. No one has asked on who the man is.
Roboute doesn't think he would be able to tell them.
The Imperium forgot what his father looked like. They saw him as bones and decaying flesh on a throne and not a man in his prime. He was an Idol and not something that once lived and walked amongst them.
Roboute looked at that face then at the one dataslate that was used for recreation.
He drew that face on the dataslate. And, he had been working quite a bit, hadn't he? There wasn't much he could do stuck in the armor as he was, but maybe he could add to his wall?
Not another face of his father, there was enough of his dead form floating around, no he would draw someone else. Someone the Imperium forgot what he looked like.
He had nine brothers who turned on the Imperium.
Nine brothers that for ten thousand years; what they were before was lost.
Fulgrim was half snake, Mortarion was a moth who doesn't have a mouth, Magnus looks the same only with more feathers, Lorgar and Perturabo have been missing, and Angron was a beast.
Horus, Alpharius, and Konrad, were dead... It wouldn't hurt would it? If he draw the faces of the brothers he knew and not just them as they were now.
Roboute reached for his reacafe a took a long drink as he got ready to sketch.
The brother that gave him this cursed future to live.
His hands were steady with every stroke and line.
It had been Fulgrim that had taught him to draw humans. He had drawn the lands of Macragge and the planets of Ultramar but people had never been of interest. Till, of course, Fulgrim had saw him paint and had asked him to join him. Sanguinius, when it was just him, Lion, and Roboute, on Macragge.
Roboute had just sketched out what would be Fulgrim's head shape and where his hair with drape over his shoulder when -- pain.
No, not pain. It was a heat. A burning heat like standing too close to a furnace or boiling pot. It started low in his stomach and climbed up to his throat. It was a fireball racing down a hallway with acid being left behind to eat what the fire did not.
His lungs burned as his throat closed and breath was stolen away from him.
His eyes watered burned and there he sat at his desk with no breath and fire and acid tearing through him. His hearts both raced and ached. Faster and faster.
All he did was sit.
He had felt death before.
He knew what was coming.
He looked over at the painting of his father. The Emperor of Mankind, the man that granted him Regency, and looked at in the eyes it. As much as he could with darkness coming.
A game, oddly come to his mind, a game that the children of Ultramar played. A tag in and tag out game. A child run and tag someone in to take their place in line or on the front.
The Emperor tagged Roboute in as Regent and now?
Roboute tagged him back in.
You're it.
I'm sorry.
Then the darkness that had been dancing in the edges of his eyes, took his sight, and then he knew no more.
Death by poisoned recafe. Not a fun way to go. What kind of poison was it? I have no idea! 40k Poison and we will leave it at that.
So how does he time travel?
When a Primarch dies it's like an explosion of the Warp right? And every Primarch is a psyker on some level. So the idea: The last thing Roboute sees is a drawing/painting of the Emperor in his prime, the last thing he thinks about was when he was younger and watching the children of Ultramar play and the Emperor. And before that he was thinking about his brothers before Chaos came for them.
He was thinking of the past.
When he died and the explosion of the Warp happened. His mind and soul was sent back in time. To the time where he was think of and frankly, the most happy. He sent back to his father and brothers! His Father may have had a part in it but shush! On that!
Now the onto what is causing the issues with his health!
The Poison doesn't follow him to his younger body, clearly. But the effects of it do. The pain, racing heart, and the like. When he ends up in the pass, he powers through it because what doesn't kill you makes you stronger and more pissed. But that adds stress to his boy, a body that has to get used to the mind of man that was years older then what it was. So couple in the lack of selfcare that he had for himself and all that. (The Depression)
So one day his body was pushed the limit and it just -- gives out on him.
And that brings us to Mortarion!
With Mortarion there is three things: one, I have not read any of Mortarion's books, two, what I know is nothing more then Fandom and quick google searches, and three, a lot of headcanons. So with that: Mortarion, my take.
Mortarion has experience with medicine, poisons, and everything of the like because of his home planet. That would make sense right? Let's say he keeps up with all the new updates on medical side of things.
One day, Mortarion is having issues in bring a planet to compliance, Orks, other Xenos, or humans that are fight back, it doesn't matter. What matters is that Mortarion calls for back up from the closest Legion.
That Legion is the Ultrmarines.
The Ultramarines come to his aid but they hide Roboute way on the ship. Like he doesn't come down to planetside or even answers his Vox. The Ultramarines are led by the first and second captain. And they are good in redirecting and just not answering Mortarion when he comes asking about Roboute.
"And my brother?" Lord Mortarion's voice was both muffled and clear through the every present cloth that was wrapped around his face. He was in his armor, much like everyone else, but his helmet was off and he was looking down at them.
When it came to Primarchs it was easy to be looked down upon. Whether from their height or because of the justification of being superior. It was there in his voice and stance: the demand for answer and expectation of one.
His father may be taller, he may even have moments were his 'superioroty' may get to him; but there wasn't a moment that his love was doubted. He heeded them in a way that other Primarchs did not with their own sons. He saw them -- or he used too.
Now, he see those who are not there.
"Father is working on the logistics, my Lord."
Lord Mortarion huffed. "Logistics." he said it like it was a curse.
We'll say that after that there is tension between the two Legions but they work together. The Ultramarines are keeping what is happening to Roboute under wraps but they can't do that for too long.
He's getting sicker and sicker. Doesn't leave his desk or take off his suit of armor, and barely eats, drinks, and doesn't touch his recafe, no matter the time, flavor, and brand.
One day; it all comes to head.
Marius hadn't been on the Macragge's Honour in what felt like years; but was just a month at most. A long, grueling, dragging, month in which he had to deal with the Death Guard and their father.
Normally he had no issue with dealing with them. His cousins that is. His uncle on the other had -- he will be happy to take a break from him. That was all he would say that matter.
Marius was standing in under the command tent as they -- Mortarion, Typhon, and himself -- discussed the ending of the campaign. There was table in front of him and on that table was a map with figurines used as marker for where the different forces where located.
The Ultramarines were just blue cones, the Death Guard were a dust color, and the enemy forces were black.
Marius grabbed a blue cone, "We can hold the ridge. In fact, it's most likely they will try a last stand here." He pointed to a valley, or where a valley should be, on the map. "If they do -- we can knock the valley walls into them. Control explosions and cause a small landside." The map was torn and faded after many days in the sun before being moved under the command tent. The acid rain had gotten to it in a few places, but it worked.
"We can route them there." Typhon moved his own pieces.
"If they are smart; they will notice that they are being led into a trap." Lord Mortarion spoke up. He had been silent up till then. His eyes, the only part of his face that was seen, was locked on the map.
Typhon laughed, "Have they been smart up to this point?"
*Buzz Buzz*
Marius looked down. His vox was going off and the caller of the Macragge's Honour.
He answered. "Report." He didn't bother to put it to his ear and left to on speaker.
He should have.
"Gage! Get back here now! It's Father! He's seized!"
What?
"What?"
"Just get back here! Father's has blood coming out his mouth and I can't tell if he bit his tongue or not!" The vox turned off.
Marius started at it before looking up.
Lord Mortarion met his eyes, "What was that?"
Marius has to explain what has been going on with Roboute and Mortarion demands to go to see Roboute himself.
He finds Roboute sitting on the floor, waving his sons away, and overall not working with them, not doing as they ask, and refusing to be treated.
"I'm fine." His brother spoke his voice that had not touched water.
He sat against the wall dressed in amour with blood dripping down his chin. His skin was pale. Paler then it normally was. There was a trimmer in his hands and when he was dragged up into standing by two his sons -- his legs were weak and his head rolled.
"Brother," Mortarion said as he stepped forward. Captain Gage had raced to his father's side.
Guilliman looked up him. "Mortarion?" He blinked and shook his head. "No, No," he paused again. "It's nice to see you, brother." He shook his head again and swayed as he let go of one of his sons' shoulders and grabbed his head. "It's better this way, isn't? Meeting like this?"
Guilliman's eyes were wild and haze and a red flush was taking over his face.
Mortarion turned to the Ultramarines. "Explain."
They explain all that they could about Roboute's health. Where Roboute was fighting them all the way to the Med Bay, with Mortarion with them, it was now much easier to move him around.
And it wasn't because Mortarion was taking his weight when his legs gave out. It wasn't because he helped get him out of his armor.
No, Mortarion did nothing but stand there in the background watching and studying everything that they did and didn't do with Roboute.
Tests are run and blood samples are taken.
Mortarion looks over the results and well...
He turned on his heels and walked to the Astropath. It wasn't a slow walk but it wasn't running. No, he would not run on Guilliman's ship, but he would waste no time.
He walked into the office with no care.
"Tell the Emperor to come to Macragge's Honour. This is not up for debate." There was a pause. "Contact the nearest Legion and tell them that they needed to be here yesterday." He turned away from the astropath, knowing his message will be sent, and returned to Guilliman's side.
Guilliman was laying in bed. A fever had taken hold and refused to abate. His breaths were puffs and he looked small. Out of his armor and in the soft clothes of Med Bay's robes.
Mortarion stared before turning to Gage. "Get a bucket and rags. The water in the bucket must be lukewarm. Put it on the warmer end if you have to, do not bring me cold water."
"Sir?"
"Your father's tempature needs to be brought down. The fastest way to do that is either a bath or a rag." He rolled up his sleeves and put the back his hand to Guilliman's forehead. "And. I think neither of us will be happy to bath."
Mortarion becomes the only who can treat Roboute.
It's the good ol'fashion 'I know you will hurt me but you are the only one I trust with this' moments.
Roboute trust Mortarion to handle his care even when he's asleep because that is his brother. Mortarion might be one of Nurgle's but that doesn't matter in his eyes. Mortarion is his brother and he knows that Mortarion will poison him, can give him so many illnesses, but he doesn't care.
Mortarion, a brother, is with him and that is enough.
He woke up to an I.V. Port in his arm with an I.V. being in it dropping clear liquid into him. The Port It itched and his wrist ached from where it was hanging off the side of the bed.
He went to scratch it.
"Don't you dare, Guilliman." His brother, one of those long stolen away by one of the Four, walked over to him. In his hand was a rag.
His brother laid it across his forehead. "It took far too much time in finding a good vein."
"Brother?" There was something off. Something not right.
Mortarion paused, "Father's on his way. Then I'll leave to him."
Mortarion wouldn't leave him to the Emperor, even if he could get off throne, and he should not be looking like that. Mortarion would call the Emperor 'father'.
"Oh," he said. That was what was wrong. "This isn't real."
I will end this here before it get any longer. But after this is Russ and then the fun part of all the Primarchs on Terra.
Oh! I came up with a name. 'Winds of a Butterfly'. The Butterfly Effect AU.
Thoughts? Oh and this is just a part one. Everyone can thank cryptids-and-muses for the all the ideas and extra part.
@cryptids-and-muses here's part 1 of notes!!!