"Gasp!"
Flavius spasmed upward, his lungs burning as if he'd inhaled a cocktail of industrial waste and poisonous fluids. He wasn't in his bed. He wasn't in the 21st century. His eyes were wide, his heart beat like a gazelle, and his lungs expanded and constricted in quick succession.
The air was a thick, yellow-grey soup that tasted of rot and ancient grease. Above him, a vaulted ceiling of rusted iron dripped black ichor, and the distant, rhythmic thrum of massive atmospheric scrubbers vibrated through the very floor. It sounded like the heartbeat of an ancient giant from mythology.
"Where... what?" he wheezed, clutching his throat.
Memory hit him like a Bolter shell. The swirling silver concoction. The Essence of Many Masks: A Meta Essence CYOA. The promise of power... and the fine print that apparently included a one-way ticket to the most miserable universe ever imagined, and which seemed to be very real.
"Fuck..." he cursed.
His greed... that was what he got for it. The tradeoff for power was this...
"Medusa," he whispered, the name surfacing from a deep well of lore-knowledge and memories of the body he now had. "I'm on Medusa. The Iron Hands' home world? Or not? Wouldn't that be Medusa IV? I'm in the Underhive."
Falvius had transmigrated into Warhammer 40'000. It was a glorious male fantasy that everyone agreed should only be enjoyed as a tabletop game or as reading material. But not the destination for your isekai story. Yet, here he was, right in an Underhive, which was the lowest level of a Hive City, a country-sized city of the Grim Dark Future.
It was dangerous. Literally everything wanted to kill you. Warhammer was the place where 'might makes right' was the highest rule to live by. Those with power either lived comfortably or miserably, but those who did live comfortably were the 10^-10000e percentile of humans. And even they were subject to intrigue, assassinations, and the overall dangers of the galaxy.
No one was safe, and no one was allowed to have nice things.
So here he was, Flavius, the biggest outliner ever. For he had at least one nice thing: The Essence of Many Masks.
This sweet drink had granted him the power to summon masks known throughout all of creation. Whether real or fictional, it didn't matter. This could also be things that acted like masks. As long as it covered parts of his face, a big enough part of his face, it counted. So, no hats and also a single eyepatch wouldn't cut it. But a pair of glasses, as long as they were large enough to cover parts of his face, counted.
And once he summoned the mask and wore it, he gained any and all powers, skills and abilities associated with the mask and or its original wearer. Sadly, that also included weaknesses. However, any form of possession or corruption associated with a mask will not affect him, so that was a bonus.
So this granted Flavius large amounts of abilities, skills and powers he could use that would definitely help him.
A hollow clatter echoed from the shadows of a waste-pipe alleyway. Three figures emerged, their skin an almost translucent, sickly pale white, draped in patchwork rags and reinforced with rusted scrap metal that was supposed to be augmetics. One had a mechanical eye that whirred and screeched, releasing sparks; another wielded a jagged 'ripper' pistol.
"Lookit this, boys," the leader sneered, his voice a gravelly rasp. "Fresh meat in the sump. Nice clothes. No augmetics. He'll sell well to the corpse-starch vats."
The shock settled as a cold spike in Flavius's chest, but beneath it, a new instinct hummed. A library of faces existed in the back of his mind, thousands of them, and their facial wear or masks waiting to be summoned and worn.
He needed protection and strength.
He reached into the "nowhere" of his mind and summoned the first mask that came to mind.
...
In a flash of light that tasted of static, a massive, ceramite-thick helmet appeared in his hands. It was cobalt blue, adorned with a white "U". It was the Mark X Tacticus Power Armour of the Primaris Marines, specifically that of the Ultramarines chapter. Without hesitation, Flavius slammed the Mark X' Tacitus Helmet over his head.
The transition was violent, but not hurtful.
His vision burst into a tactical HUD. Red runes flickered, identifying threats, calculating wind speed, and highlighting structural weaknesses in the gangers' rusted armour. But it wasn't just the senses and mind. His muscles, changed physically, grew more powerful and denser, as did his bones and his entire biology.
He had grown larger, his muscles had formed, turning him into something beyond peak human limits - A literal bioengineered warrior. He settled into a new 'state' with the terrifying, practised efficiency of a centuries-old bioengineered Primaris Space Marine.
He knew hundreds of ways to kill a man with his bare hands, and his reflexes had accelerated to a blurring peak for ordinary humans.
"What's that on 'is head?" the ganger yelled, leveling the ripper pistol.
"He grew large!!"
Flavius didn't answer. One second he was in front of them, and in the other he wasn't.
To the gangers, he was a blue blur. He caught the first man's wrist holding the weapon, grabbed his skull, and with the casual strength of a 2,5 meter monster Primaris veteran, he snapped the neck like a dry twig. He didn't need a Bolter; he himself was the weapon.
A roundhouse kick, perfected over thousands of combat simulations, shattered the second man's thorax entirely, sending him flying into a vat of chemical waste and splattering his remains on the rusted wall.
"Throne! He's a freak! A witch!" the leader screamed, scrambling back.
Flavius felt the cold, stoic resolve of the Ultramarines' mindset—his unbreakable will. There was no fear. Only the mission. But as he stepped toward the leader, a thought struck him: This might be too loud and could get attention.
In the Hive City, the Adeptus Arbites, the planetary police force, or the Inquisition, will find a 'Blue Helmed superhuman' who wasn't officially an Astartes, in mere hours and hunt him down for heresy and chaos corruption.
Then again, no one would tell if there was no one to tell.
Still, for testing's sake, Flavius willed the helmet to be unsummoned. As soon as he willed it, the Mark X helmet vanished into smoke.
The leader blinked, confused by the sudden disappearance of the helmet and the shrinking of Flavius, but before he could raise his weapon again and catch his bearings, Flavius summoned a different mask.
The sleek, promethium-tough weave of Deathstroke's Ikon Mask. Slade Wilson, the Deathstroke of DC Comics.
The leader fired. The solid slug tore toward Flavius's shoulder, but the world had already slowed to a crawl once again.
The shift was electric. Slade Wilson's mask provided lethal genius, his tactics, and all the abilities granted to him by his special meta-human body. Flavius felt his brain overclock; his senses increased by a thousandfold. Every nerve ending sparked with the predatory instincts of the DC comics' greatest assassin.
He saw the bullet and predicted its trajectory based on the angle of the ganger's trembling wrist.
With a twitch of his head, mere millimetres, the slug whistled past his head, buried itself in a rusted pipe, and hissed as steam escaped. Flavius moved forward with such fluidity that it seemed like he literally flowed. Once again, he was a blur of motion.
"Wh-what are you?!" the leader shrieked, firing wildly.
Flavius closed the distance in a heartbeat, his hand snapping out and catching the slide of the ripper pistol mid-cycle, jamming the mechanism with his thumb while his other hand delivered a palm-strike to the ganger's sternum.
He hadn't used all his strength, but was precise. He felt the man's heart rhythm stutter under the impact and stop.
The remaining ganger who had his arm broken badly charged, swinging jagged lead pipes. He was heavy on those drugs, which explained his lack of reasoning in this situation.
Flavius spun, his vision tracking both targets simultaneously. He saw the seemingly endless flaws in the man's movements, the hitch in his breath, the exact moment his muscles tensed to strike.
He moved between strikes with ease, making the man look like a toddler. A kick to a kneecap here, a chop to a carotid artery there. In a single second, the alley went silent.
Flavius stood over the dead bodies; his single visible white eye was glowing with a cold, predatory look. He didn't feel like a hero or a monster; he felt like a professional. They were fools who tried to kill him or sell him to become Corpse Starch... they deserved what they got. He reached down to strip the leader of his little valuables and a jagged blade.
Sadly, he didn't get the equipment with the masks. Only the masks with the powers, skills and abilities. But that wasn't a big problem; he already had a way around it.
.
Flavius sat on a crate of discarded machine parts, Deathstroke's mask resting in his lap. His body was fully healed, thanks to Deathstroke's healing factor. The shock of his new situation had finally begun to settle, giving way to a grim, pragmatic acceptance.
He was in the Underhive of Medusa, a world that loathed weakness and worshipped the strong. He was in Warhammer, the grim dark future. That wasn't going to change. It would be better if he... although, maybe he could change that. He needed more time to research his new powers.
"Right," he muttered, wiping soot from his brow. "Rule one: Don't trust anyone. Rule two: Find a way out of the Underhive. Rule three: Lie, betray, kill whoever you have to to survive."
He spent the next few hours playing around with his mask.
First, he summoned one of, if not the most iconic, masks ever. He was confused why he hadn't summoned it sooner.
Batman's mask.
He could summon any one of his many versions' masks. Every time he switched, the reflexes and skills would disappear, leaving him feeling strangely vulnerable and naked. Batman's masks gave a feeling of being ready, but also troubling paranoia.
Eventually, Flavius settled on Batman 1 Million's mask. It was, in his humble opinion, the best one. And there were several reasons for that.
He came from the 853rd Century. As such, he had an 853rd-century physiology, which meant a number of capabilities more developed than those typical of the 20th or 21st Centuries. Access to 853rd-century education allowed Batman to create technology beyond anything available in the 21st Century, and made even advanced technology from earlier periods seem simple.
Apart from the usual Batman skills, he was a serious genius. He was noted to have an IQ of 1045. And if there was one thing that Flavius could use in Warhammer 40k, it was intelligence and the education and genius to understand and improve the stagnating technology of the Imperium of Man.
Now, Batman's mask was, of course, iconic, but if intelligence was what Flavius wanted, there were many more he could use as well.
Also from DC Comics, there was Michael Holt, AKA Mister Terrific, a brilliant inventor and businessman.
Holt was described as having "a natural aptitude for having natural aptitudes," picking up complicated skills quickly and retaining them, with an IQ of 179. Holt has been described as the third-smartest person on Earth, after Lex Luthor and Bruce Wayne.
That was Earth-616 of the 21st Century and not the 853rd one. So Bruce Wayne definitely took the top spot.
Leaving the DC Universe and moving to its brother, Marvel Comics, there were many geniuses for Flavius to choose from.
Tony Stark, AKA Iron Man; Victor von Doom, AKA Dr Doom; Reed Richards of Earth-1610, AKA the Maker and Mister Fantastic; T'Challa, the King of Wakanda, AKA the Black Panther; and Hank Pym, AKA Ant-Man, to name a few.
Out of these, Dr Doom and The Maker from Earth-1610 were especially interesting. Dr Doom could use magic and was a genius inventor and creator of technology, often mixing the two. And the Maker was a serious genius with Mister Fantastic's special biology, which could stretch.
Now, there were a few things that Flavius was thankful for, and he was glad he had tested them out before they might become important.
For one, he gained all the powers, abilities, and skills that came with the mask or the person associated with it. BUT, his body always stayed human.
He tested that out by summoning an Eldar Wraithbone helmet. His senses, agility, and speed improved, and especially his psychic powers shot through the roof. He also grew taller and leaner, but he still looked like a human and didn't turn into an Eldar.
Flavius also summoned a T'au combat armour helmet and confirmed this. He grew smaller, lost all his psychic powers, but remained human.
The final mask Flavius got familiar with was one of, if not the most iconic, villains: Darth Vader.
The Lord of the Sith and the Chosen One of the Star Wars Universe was a very strange experience. Now, because any form of possession or corruption associated with a mask didn't affect Flavius, he wasn't trapped like Vader was, but he was still filled with rage and pain.
There were both positives and negatives to this.
On the one hand, he didn't gain Vader's memories. But on the other hand, all that rage and anger drew the attention of a very, very, VERY dangerous being: the Chaos God Khorne.
Flavius immediately took the helmet off again. He took a moment to calm himself and then finally thought about his next steps. This had been incredibly dangerous. So he had to truly think about doing that in the future.
It was time to leave the Underhive.