Cold.
That was the first sensation.
Not pain. Not fear.
Just… cold.
As if his entire body had been submerged in ice water, nerves frozen, lungs refusing to breathe.
Then came the sound.
A slow… rhythmic beeping.
No — not a machine.
A heartbeat.
But it wasn't steady.
It was violent.
Like war drums echoing inside his skull.
"Is he alive?"
A distant voice.
Blurred. Distorted.
"AB+ confirmed."
Another voice — sharper, clinical.
A pause.
Then laughter.
Not joyful laughter.
The kind reserved for corpses that hadn't realized they were dead yet.
His eyes snapped open.
Light stabbed into them like needles.
He gasped — but the air tasted wrong.
Metallic.
Thick.
Blood.
He was lying on a circular stone platform carved with glowing crimson symbols. The chamber around him resembled a cathedral fused with a laboratory — towering pillars, chains, ritual circles, and armored guards standing like statues.
Above him floated a crystal sphere filled with swirling red liquid.
Blood.
A robed priest leaned over him, holding a thin obsidian blade.
"Subject conscious. Unexpected."
The priest turned his head toward a balcony above.
There, nobles sat watching — dressed in colors that shimmered like liquid metal. Their eyes held no warmth.
Only evaluation.
Like merchants inspecting livestock.
"State your awareness," the priest commanded.
The boy — no, the man inside the boy — tried to speak.
His throat burned.
"…Where… am I…?"
His voice was smaller. Younger.
Wrong.
He lifted his hands.
They weren't his.
Slimmer. Pale. Marked with faint crimson veins glowing beneath the skin.
Memories slammed into him.
A truck's headlights.
Rain.
Screeching brakes.
Then darkness.
I died…
The realization settled with terrifying calm.
I was reborn.
Before he could process further, iron clasps snapped shut around his wrists and ankles.
"Blood Ascension Ceremony complete," the priest announced. "Proceeding to Hematype Revelation."
The crystal sphere descended.
A needle of condensed blood extended from it and plunged into his chest.
Agony.
Not surface pain.
It felt like his bones were being drilled open and filled with molten iron.
He screamed.
The chamber echoed with it.
Symbols ignocated across the floor.
The sphere flared.
Then—
It stopped.
The pain vanished instantly.
Too instantly.
Like a guillotine blade falling silent.
The sphere dimmed.
Its crimson glow flickered… then turned murky gray.
Murmurs spread through the nobles.
The priest frowned.
He placed a hand over the boy's heart.
His expression twisted.
"…Impossible."
He raised his voice.
"Hematype result — AB positive."
Silence.
Then—
Laughter erupted across the chamber.
Mocking.
Disgusted.
Relieved it wasn't them.
"Defective blood."
"A waste of ritual resources."
"Dispose of him."
The boy lay there, chest rising and falling, mind racing.
AB+…?
Even in his past life he knew blood types.
Universal receiver.
Compatible with all.
So why—
A memory surfaced from the body he now inhabited.
School.
Children lining up for blood testing.
Results being announced.
Cheers for O−.
Respect for A and B.
Pity for AB−.
Then—
Him.
AB+.
Teachers whispering.
Parents looking away.
Other kids laughing.
In this world…
Blood type wasn't medical.
It was destiny.
The priest wiped the blade clean as if the boy were already dead.
"No combat affinity. No mana resonance. No elemental alignment."
He turned to the nobles.
"An F-Class Blood."
One noble waved dismissively.
"Send him to the Frontier Blood Fortress. Let him die usefully."
Chains released.
Guards dragged him off the platform like discarded meat.
As they hauled him through the cathedral halls, he saw murals lining the walls.
Warriors cloaked in crimson aura.
Mages wielding rivers of blood like weapons.
Elemental storms born from veins.
At the center of every mural stood figures radiating overwhelming power.
Their sigils marked clearly:
O−
O+
A
B
Never AB+.
Not once.
They threw him into a transport carriage with iron bars.
Dozens of others were inside.
All thin.
All broken.
All bearing the same sigil carved faintly into their necks:
AB+
One boy whispered beside him.
"First time?"
The MC nodded weakly.
"Don't worry," the boy said with hollow eyes. "We die fast at the fortress."
The carriage lurched forward.
Outside, the capital city stretched — towering bloodstone spires, floating crimson crystals, knights flying with aura wings.
A world of power.
A world he had been reborn into…
At the very bottom.
Hours passed.
By nightfall, the carriage reached a colossal wall of black iron — the Frontier Blood Fortress.
Beyond it lay wastelands crawling with monsters.
Disposable soldiers were stationed here.
The guards shoved them out.
A commander approached — towering, radiating violent crimson aura.
His sigil burned bright:
O−
The air around him trembled with raw killing intent.
He looked over the prisoners like tools.
His gaze paused on the MC.
"AB+?"
He spat on the ground.
"You're not soldiers. You're blood bags."
The prisoners were forced into labor immediately — hauling weapons, cleaning monster carcasses, carrying wounded elites from battle.
Hours turned to days.
Beatings were routine.
Food was scraps.
Sleep was optional.
AB+ weren't considered human enough for mercy.
But the MC watched.
Learned.
Analyzed.
Even while starving, he studied the elites' combat styles… aura flow… regeneration patterns.
His past-life analytical mind refused to die.
Then came the night that changed everything.
Sirens blared across the fortress.
Crimson flares shot into the sky.
"Monster breach!"
Walls shook.
Explosions thundered.
A colossal beast smashed through the outer gate — a multi-limbed titan with bone armor.
Elite soldiers charged.
Aura clashed.
Blood magic ignited the sky.
The MC was assigned to stretcher duty near the battlefield.
That's when he saw him—
The O− commander.
The same one who spat at him.
Now lying on the ground…
Chest torn open.
Heart barely beating.
Even his monstrous regeneration was failing.
He was dying.
Healers surrounded him, panicking.
"We need immediate transfusion!"
"From whom?!"
"All O− units are down!"
Silence.
Then—
A healer's eyes slowly turned toward the stretcher slaves.
Toward the AB+ prisoners.
Disposable.
Compatible with all blood…
In theory.
But in this world, transfusion meant death.
Absolute rejection.
Bodies exploding from Hemophage Curse.
The healer hesitated.
The commander's pulse faded.
"…Use him."
A finger pointed.
At the MC.
Guards seized him before he could react.
Dragged him beside the commander.
Strapped him down.
"You should be honored," a healer muttered. "Your blood will serve a superior before you die."
Needles plunged into both their chests.
Blood began to flow.
Crimson light surged violently through the tubes.
Pain.
No — this was beyond pain.
It felt like foreign fire invading every vein… trying to devour him from inside.
His vision turned white.
He heard bones cracking.
Healers stepped back.
Waiting for the explosion.
"Rejection in 3… 2…"
But it didn't happen.
Instead—
The crimson light changed color.
It began mixing with a strange silver hue emanating from his own blood.
The ritual circle malfunctioned.
Symbols spinning out of control.
Wind erupted across the chamber.
The commander's wounds began regenerating rapidly.
Faster than before.
But at the same time…
The MC's body lifted off the ground.
Veins glowing violently.
Aura igniting for the first time.
Deep.
Savage.
Primordial.
A sigil burned into existence above him.
Not AB+ gray.
But—
Crimson.
O− crimson.
Healers stared in horror.
"…He's not dying."
The MC's eyes snapped open.
Now glowing berserker red.
A voice — ancient and feral — echoed in his mind:
"Foreign blood accepted… Adaptation complete."
The transfusion tubes shattered.
Aura exploded outward, blowing soldiers off their feet.
The weakest blood…
Had just survived the impossible.
And somewhere deep within his veins—
Another heartbeat began.
Not his own.