"Why are you back?"
Nine days after the Battle Sisters had departed, Yuki stood in front of a fully armed Steff, looking utterly bewildered.
"A temporary follow-up mission requires your assistance. As someone with established ties to you, I was reassigned here."
Yuki felt a pang of unease. Looking at the Battle Sisters accompanying Steff this time, there was no trace of the green recruits from before. Their faces bore the weathered marks of countless battles. With shaved heads, topknots, and discarded helmets, these women were the elite of the elite among the Adepta Sororitas.
"This is the Seraphim Squad. These two are the Palatine and the Icon Bearer."
This was a far cry from the "line-filler" units that were comparable to Imperial Guardsmen. The Seraphim were the elite shock troops of the Battle Sisters, equivalent to Terminator veterans among Space Marines. The few dozen Seraphim standing before him now were worth more in combat than the hundreds of sisters from the previous mission combined.
Yuki had felt a sliver of melancholy during their earlier parting, but seeing them return so quickly, he didn't know whether to laugh or sigh.
"What exactly is the mission?"
"Alien lifeforms have reportedly been sighted in the Underhive junk heaps. Both the Ecclesiarchy and the Adeptus Mechanicus are very interested. Word is that other factions are eyeing them as well, so we need to secure them first."
The Underhive junk heaps were essentially the treasure troves of the Warhammer world; almost anything could turn up there. It was like the Underhive gangs had a pocket like Doraemon's—anything could escape from it. If someone like Eleven, a near Alpha-level psyker, could appear here, then nothing was impossible.
"Fine. It looks like I'll be busy for a while."
Honestly, Yuki was more interested in exploration than administration. However, his current identity was the partner of the Great Devourer, and he felt the need to prepare a gift worthy of her status. Consequently, he had spent the last two years handling Underhive politics, never getting the chance for a proper expedition. Since things had been quiet lately, he figured he could take Yuno along to relax a bit.
"Let's move."
...
The Junk Heaps.
A massive canyon buried under mountains of refuse, it was one of the few geographical features that hadn't been fully terraformed. Over millennia of dumping, it had evolved into a unique and treacherous ecological zone.
"Do you have a specific description of what you're looking for?"
Steff hadn't received detailed instructions either. "I only know it's an ancient object, similar to a holy relic. The 'Cogs' found mention of it in historical archives; they likely know more than they're letting on."
Yuki was speechless. The Ecclesiarchy was clearly acting on a whim, rushing in just because they heard a rumor of "something" being there.
"I'll do my best to help you find it. But if we come up empty-handed, don't blame me."
With that, Yuki arranged for the group to stay at a local settlement. The local power-broker, hearing that the boss of the Yuki Gang had arrived, showed due respect and provided quite luxurious accommodations.
The group settled in and began studying the local terrain. According to the intelligence the Ecclesiarchy had siphoned from the Mechanicus, the suspected location of the alien creature was in a no-man's land about ten kilometers away. It was a trash-dumping vent with winding tunnels that supposedly led all the way outside the Hive City.
This planet had originally been a habitable world, but after ten thousand years of exploitation, the environment had been utterly devastated. Now, all residents were forced to live within the Hive Cities to survive. Yuki began to understand why the Mechanicus wanted to capture this creature. To have survived undetected meant it likely lived outside the Hive for long periods. If it could endure the harsh surface environment, harvesting its genes could theoretically allow humans to expand their range of activity on this planet.
The Ecclesiarchy likely shared this thought.
That night, everyone drew up a detailed scouting plan. Yuki and Yuno would not be joining the front lines; after all, the image they projected to the world was that of a frail couple with no combat capability. Steff had only brought them along to make navigating the Underhive easier.
Yuno didn't mind. There wasn't a biological organism she hadn't seen; for her, this trip was just a way to spend time playing with Yuki.
"I heard there are strange wild animals frequenting this area. Want to go take a look?" Yuki wanted to get closer to Yuno and understand more about her.
In the past two years, their relationship hadn't changed much. Yuno enjoyed staying close to him and teasing him, and occasionally she would tell Yuki stories about her past. The Tyranids had already destroyed several galaxies, though none were as enticing as the Milky Way. Due to the limited imagination of a certain "Nottingham-based enterprise," the Milky Way was the only place in the universe that could interest the truly powerful entities of this world.
In Yuno's own words, the galaxies they had consumed previously barely covered the energy costs of the hunt. They had expanded their scale slowly by consuming rogue planets and small star clusters along the way.
"So the Milky Way really is a treasure chest. The harvest from the Hive Fleets I've sent out so far has exceeded the losses."
Yuki was confused. "But haven't the fleets sent here mostly failed? How can there be a profit?"
"Silly. You think we only eat biomass? You think we use every scrap of biomass we capture just to spawn more soldiers? In reality, a portion of the digestion pools captured by the Hive Fleets is used for fleet expansion, but the majority is sent back to the main force."
Yuno spoke while nibbling on a snack. "The main force is currently traveling at the speed of light. However, at this rate, it will still take two hundred years or more to get here. The daily energy consumption is staggering."
Yuki grew even more perplexed. "Then... the fleets can't rely on biomass alone, can they?"
"Of course not. That's why we sometimes have to stop and eat a few stars to recharge."
"That sounds like something the Necrons would do."
"It can't be helped. It's inefficient, but it's better than nothing."
As she spoke, Yuno seemed to recall the consumption operations she had personally deployed. "This requires specialized Hive units that have never been used in the Milky Way, collectively known as 'Star-Devourers.' They are only about the size of a human finger. It takes trillions of them to withstand the immense energy of a star. Specialized organs in their bodies perform a type of photosynthesis, converting inorganic matter into organic matter. In just a few months, they can completely hollow out a sun."
Yuno frowned, seemingly remembering something unpleasant. "But those bugs are the most fragile. They have no combat capability; even a human infant could crush them easily. That's why they need massive Hive Fleets to protect them. And during the conversion process, they're practically like bombs. I nearly lost a tentacle because of them."
Yuno looked at Yuki. "Strictly speaking, I can create any kind of organism. Should we try to catch a Custodian later and see what happens?"