Warhammer 40K: System Error, I Summoned the Arch-Traitor!

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The lounge was relaxed. Easy.

But before the two brothers could exchange more than a few words, the door swung open.

Sanguinius strode in, Kaelen a step behind him.

"Our good friend Kaelen brings new tidings, brothers."

The Great Angel came straight to the point, his tone carrying a thread of amusement.

Horus raised an eyebrow and set down his glass.

"What tidings?"

Sanguinius didn't rush to answer. He lifted his chin toward Kaelen, meaning obvious.

Kaelen didn't stand on ceremony. He crossed the room and dropped into the seat directly across from the two primarchs.

Sanguinius settled gracefully beside him, his posture like a swan with its wings folded in.

Horus, reading the room, reached over and picked up the bottle from the table, pouring two glasses of amber liquor without being asked.

Kaelen grabbed his glass and took a sip. The liquid slid down his throat, hot and sharp. He stared at the amber in his glass for a few seconds before he spoke.

"I just used my ability to pull a soul fragment."

Lion and Horus looked at each other.

Their gazes met for a fraction of a second, then both landed on Kaelen.

Horus asked first. "Whose?"

"Fulgrim." Kaelen paused, as if choosing his words, or confirming something to himself. "Not the fallen Phoenix Lord we all know. This soul fragment comes from a different parallel world-line."

He lifted his head and met their eyes directly.

"A completely different world-line. You follow me?"

"A variable appearing at a specific point in time?" Lion's voice was low, his tone eerily calm. "Like the alternate histories the ancients of old Terra used to theorize about — a key figure makes a different choice on a given day, and a chain of butterfly effects follows?"

Kaelen blinked, then caught up.

Right. These were primarchs. In terms of raw intellect and breadth of knowledge, they were operating on a level he couldn't even begin to measure.

He nodded. "That's the idea. And in that world-line, the two sides of the Heresy were completely reversed."

"Oh?" Horus leaned forward slightly.

"The Fulgrim of that world-line was a loyalist." Kaelen said it one word at a time. "He never picked up the Laer Blade. From start to finish, he was a faithful warrior of the Imperium — and was later venerated as a Living Saint."

Contrary to what Kaelen expected, neither Horus nor Lion looked particularly shocked.

If anything, their reactions were muted. Almost... interested.

As for Sanguinius, the Great Angel had already sensed Kaelen's thoughts through his psychic awareness long before this conversation started. He simply sat with a faint smile, wearing the expression of someone who already knew the punchline.

"Then who were the rebels?" Horus pressed, curiosity plain in his voice.

"Take your original history and swap the rebel and loyalist rosters." Kaelen drew a slow breath. "I'm sorry to say, Lord Lion, the rebel side in that world-line includes you."

The air went quiet.

Lion's brow furrowed slightly.

He didn't lash out. He didn't show anger or displeasure. He simply narrowed those deep, dark eyes, as if turning over a serious philosophical problem.

He didn't understand it.

He had always held to the truth that loyalty was its own reward. The idea that a version of himself would walk the path of rebellion was incomprehensible to him.

It was as absurd as asking why the sun rose in the west.

But the Lion was not an impulsive man. He pressed the confusion down and asked, his voice level.

"Who was the driving force behind the rebellion?"

Kaelen hesitated for about 3 seconds.

"Roboute Guilliman."

"...Guilliman."

Lion's expression shifted into something complicated.

Horus had the exact same reaction. The two exchanged a glance, and both let out a breath at the same time.

Well. That explains it.

Not incomprehensible at all, actually.

Horus rubbed his chin. The answer was unexpected, but the more he turned it over, the more it had that particular flavor, the kind of thing that shouldn't make sense until suddenly it did.

"What was the reason? What made Guilliman choose to betray the Imperium and Father?"

He was willing to believe his Macragge brother wasn't the type to act without cause. As a seasoned political mind, Horus genuinely couldn't find a motive for Guilliman to turn against the Imperium, unless something had happened that crossed a line he couldn't come back from.

"It concerns his adoptive mother," Kaelen said. "Lady Euten."

Tarasha Euten.

Lion frowned, reaching back through memory for the mortal woman he had met exactly once.

When he, Sanguinius, and Guilliman had been establishing Imperium Secundus on Macragge, she had been there briefly.

A respectable mortal. That was the full extent of his impression.

"If something happened to that woman, there's a very real chance Robert would do exactly that."

Sanguinius shook his head slowly, something like understanding and sorrow moving through his clear eyes.

"Not a chance." Horus shook his head, flat and certain. "A certainty. I know Robert. I know how he feels about his adoptive parents. If something truly happened to his mother, there's no way in hell he'd just endure it."

The three primarchs fell silent.

Horus was the first to break it. Kaelen's news had pulled up a memory, a conversation on Cadia, with the Necron called Trazyn the Infinite.

Trazyn had a perfect clone body of Fulgrim in his collection.

"My friend." Horus looked at Kaelen. "Are you thinking of placing this soul fragment into that clone body?"

"That's the idea." Kaelen admitted it without hesitation. "The problem is, I don't have anything valuable enough to trade Trazyn for it."

He spread his hands. Then his eyes shifted, and something clicked.

"What if I use the soul fragment itself as collateral? Tell the Infinite it has enormous appreciation potential, get the clone body up front, and then, once it's done..." He let the pause hang. "Default on the deal?"

Horus stared at him for a full second.

Then he burst out laughing.

"A genius idea. Somewhat unethical, and a complete violation of the spirit of contracts."

"My friend, did you do this sort of thing often, back in the day?" Horus grinned.

"Don't slander me!" Kaelen shot back with complete moral authority. "I'm drawing on experience from observed cases. And besides, there are people out there who make me look like a model citizen, people who try to take out loans using their credit rating and can't find a single lender willing to touch them."

Far away on Terra, something on the Golden Throne's communicator emitted an inexplicable sneeze. Outside the chamber doors, the Custodes snapped to full alert as if a threat had materialized from thin air.

Kaelen and Horus traded jabs, and the mood in the room lifted.

The Lion, however, did not join in. He was working through something.

"Kaelen." Lion's voice was steady as bedrock. "Can you guarantee that this Fulgrim won't fall again?"

"Theoretically — unless the Chaos Gods personally intervene and pile on every negative influence they have in a direct, sustained assault — then a Fulgrim fused with this soul fragment would, in all likelihood, end his own life before corruption could take hold." Kaelen chose his words carefully.

He paused, then added.

"Don't forget. The Fulgrim of that world-line was a Living Saint."

Lion rubbed his chin and sank deeper into thought.

Sanguinius watched him for a moment, then spoke gently.

"I don't think you need to agonize over this too much, brother. We have several of us holding the line right now, and Father's power can manifest at any moment. If something truly goes wrong, he will be the first to know."

"That's not what I'm worried about." Lion shook his head. "I'm thinking about Fulgrim's value. His command ability is real, I won't deny that. But his character flaws are equally real. As for personal combat, I'm not concerned. His swordsmanship is top-tier even among us."

He looked at Kaelen again, his gaze measuring.

He knew about this kid's track record.

Even Konrad Curze, the one who made everyone's head hurt, the one infamous for his madness, had been changed by him.

So changing a cloned brother... wasn't out of the question.

And if things went sideways, Horus and Guilliman were still there as a fallback.

Curze and Fulgrim had always gotten along, too. That might prove useful...

"Lupercal." Lion turned to Horus. "What do you think? I consider this a viable option. Bringing a primarch-level combatant back to the Imperium, clone or not, he's still a gene-primarch. At minimum, he can help shoulder the command burden of the Indomitus Crusade in the Sanctus."

Horus smiled, a sharp light moving through his eyes.

"It seems we're in agreement, Johnson."

The two primarchs looked at each other across the table. No words needed.

Sanguinius laughed softly and raised his glass. Kaelen exhaled, picked up his own, and drained it.

This was going to work.

---

(For readers unfamiliar with collateral, here's a quick example: Old Wang needs cash and owns a Porsche worth a million. He comes to you and says, lend me 800,000 and I'll put the car up as security, except the car stays with him because he still needs it. You agree and hand over the money. At that final step, defaulting is trivially easy, because Old Wang can disappear with both the car and your money any time he likes. That said, given the Infinite's nature, he won't fall for a simple pitch. You'd need the right bait to hook him. I'll be patching that part in later.)

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