Solution

When the Emperor created Gene-seed based on the Primarchs, he had already determined each Legion's traits, along with their inevitable flaws.

Genetic engineering was a profound field. Even the Emperor, for all his disregard for science, had to follow some basic rules: to gain something, you had to give something up.

Take the Gene-seed of the XV Legion, the Thousand Sons, in Hogg's hands. Since Magnus was the genetic template, their Gene-seed was destined to produce Space Marines with enhanced psychic abilities. That had been set from the beginning.

But with the gift of psychic mastery came exposure to the Warp's corruption. It was like going into the sea to catch fish—you were bound to get wet.

After the first Thousand Sons warriors were assembled, they began developing their psychic abilities on their own. But as their powers grew, a horrifying, uncontrollable degenerative mutation began to spread.

This strange affliction, known as Flesh Change, tormented the Thousand Sons. Space Marines with severe mutations could only be preserved in Stasis Fields.

In the Blacklight's laboratory, two Primarchs—one large and one small—discussed a treatment plan while facing a Thousand Sons warrior in a nearby Stasis Field. He was so disfigured that he no longer looked human.

Magnus had once tried to fuse so-called Spiritlings into Space Marines' bodies, hoping that flesh adapted to psychic powers would develop a habitual resistance to low levels of psychic energy. It was essentially an unconventional form of desensitization training.

But the so-called Spiritlings had been confirmed as daemons, so the plan had failed. Magnus had no choice but to turn to Hogg.

"My plan seems useless, so I can only ask what you can come up with."

After carefully studying Magnus's plan, Hogg held off on reaching a conclusion. Instead, he examined the Thousand Son, whose severe mutations had produced arthropod limbs and extra organs. He removed tissue that posed no immediate threat to his life for testing.

He injected the Thousand Son with a highly concentrated nutrient solution, then used instruments to record changes in the tissue around the incisions every thirty seconds.

From the test results, Hogg discovered that these highly mutated tissues were far too compatible with psychic energy. They were practically Warp creatures existing in the real world.

Prolonged use of psychic powers, combined with the wounds they sustained in battle, had caused injuries that should have healed normally to become corrupted and transform into tissue better suited to channeling psychic energy. The proof was that the more severely mutated a Space Marine was, the more powerful his psychic abilities became.

But that raised a question: other Legions had their own psykers, so why hadn't they developed mutations? There had to be a difference.

After informing Magnus of the results, the two began looking for control groups.

Soon, several psykers from different Legions were brought to the laboratory for tests. They included Space Wolves, Imperial Fists, and Blackwatch.

After comparing the psykers' data from different Legions, Hogg determined that the problem lay with the Gene-seed.

He extracted Gene-seed from the mutated Thousand Sons, then ran a series of stimulation tests on the newly obtained samples.

Anyone who had worked in a biological laboratory knew that control experiments were the most troublesome. A single overlooked variable could throw off the whole experiment.

After 7,128 control trials over nearly a month, Hogg noticed something unusual in a particular set of data.

In that experiment, the Thousand Sons' results stood out: the Gene-seed synthesis experiment.

If humans reproduced sexually, Space Marine Gene-seed reproduced more like asexual organisms. Once implanted in a body, Gene-seed would synthesize new Gene-seed after absorbing enough nutrients.

During synthesis, the Gene-seed recorded its host's combat experience while slowly absorbing energy from the Warp. This tiny amount of energy was not enough to corrupt the newly formed seed.

But when compared with the synthesis data for other Legions' Gene-seed, the Thousand Sons' seeds absorbed Warp energy dozens of times faster.

This abnormal rate of absorption had been recorded from the moment the seeds were created, making the Space Marines they were implanted in develop a pathological craving for psychic power.

On top of that, the Thousand Sons all fought as psychic spellcasters. Their highly activated psychic energy caused their flesh to mutate, making them even more desperate to use and absorb more psychic power.

Once he had identified the problem, Hogg explained his treatment plan to Magnus:

"Magnus, I have two solutions. You can choose one. First, the Thousand Sons' warriors can never use psychic powers again. Once they've mutated this badly, there's basically no saving them. Once they're caught in this vicious cycle, it's a dead end."

Magnus dismissed that option outright. Although he no longer worshiped the power of psychic abilities, he couldn't deny how useful they were.

"I choose the second option. The Thousand Sons can't lose their psychic powers," Magnus said fervently.

"Then we can only suppress the rate at which they absorb psychic energy while the Gene-seed is being synthesized. We'll also make another batch of Gene-seed using your genome. As it happens, I have the technology for that."

Soon, a Blackstone Device was delivered to the laboratory. Magnus stared at the alien contraption, its green lights flashing, completely bewildered.

"What is that? The Imperium expressly forbids the use of Xenos technology."

"Give me a break. I've been using this stuff openly for ages, and the Emperor hasn't said a word, has he?"

Watching Magnus get hung up on this, Hogg realized for the first time that Little Pony was so naïve.

"There's no 'but.' Everyone uses it; you're just the one who doesn't know. Do you think the other Legions have fewer hidden Xenos gadgets than I do?

"Honestly, I'm even more certain now that you're a 'sunburned Ogryn.' Never mind anything else—after the way you looked when you transformed the other day, you've got a nerve accusing me of using Xenos technology.

"Forget it! As Primarchs, we were never pure to begin with. Our very existence is the greatest blasphemy against the Imperial Truth. Without the Xenos technology you're complaining about, these warriors would all be left to die."

Hogg's words left Magnus speechless, and he visibly wilted. All he could do was watch as the device was installed in front of him.

Seeing Magnus standing there like a child punished for misbehaving, Hogg looked toward the utterly mutated Thousand Sons warrior in the Stasis Field. For once, he managed to keep his emotions in check.

"Little Pony, if you want them to end their lives with dignity, you have to take responsibility. I'll ask you one last time: do you accept this plan?"

At Hogg's words, Magnus seemed to understand. With visible difficulty, he nodded.

The highly mutated Thousand Son was placed inside the Blackstone Device's field. Emerald lightning indicated that the device was running smoothly. With the Warp's influence cut off, the Thousand Son, who had been enduring agony, began to relax.

After a while, the writhing mutated tissue began to shrink. Hogg took a scalpel and carefully removed it piece by piece, leaving the warrior with a roughly human shape.

Seeing that the treatment really worked, Magnus breathed a sigh of relief. As long as his sons could be saved, he didn't care if it was Xenos technology. If anyone called the device disloyal, the Thousand Sons Legion would teach him to choose his words carefully.

Hogg set down the scalpel and took out a huge needle.

"Come here. I need to collect a sample of your genes and make a new batch of Gene-seed. It's a good thing the Emperor didn't hold back when he taught me how to make Gene-seed."

Scarlet blood was drawn from Magnus's body, filling several enormous tanks.

Having finally gotten the chance to collect a sample from another Primarch, Hogg didn't hold back. He filled an entire container with blood, nearly draining Little Pony to death.

Just as Magnus thought Flesh Change could now be completely eradicated within the Legion, he suddenly realized that his son on the operating table had died.

Panicking, Magnus hurriedly pulled out the needle in his body, grabbed Hogg as he drew blood, and demanded:

"Damn it! Tell me what happened to him! He was fine just now. How did he die?"

Hogg slapped Magnus's hand away and, without indulging him, kicked him to the ground.

"Didn't I tell you? Those who have mutated this badly have only one possible end. I gave him a dignified end, so he wouldn't have to suffer through the agony of mutation anymore. What were you hoping for?"