Yellowstone was one of the supervolcanoes located on the North American continent.
According to existing research, a full-scale eruption would blast an enormous quantity of volcanic ash high into the atmosphere.
Large amounts of sulfur dioxide would then react within the atmosphere, producing sulfuric aerosols capable of reflecting sunlight even more efficiently than glaciers.
Under normal conditions, researchers estimated that the resulting "Ice Age" could take at least a decade to fully subside.
And that was under normal conditions.
It did not account for the global Ice Age that was already beginning to take shape.
Even if they considered only the eruption itself...
Ten years later, the excess carbon dioxide remaining in Earth's atmosphere could swing the climate from extreme cold into prolonged extreme heat.
And that warming could persist for thousands of years.
A true hell of ice and fire.
In theory, deliberately triggering Yellowstone would be enough to kill more than half of humanity.
As for why MEI, who had spent the past several days simulating the Herrscher of Blood's thought process, was so certain Kali would choose Yellowstone...
The reason was simple.
Yes, Earth possessed other supervolcanoes.
A handful were even larger than Yellowstone.
But at present, Yellowstone's internal system had accumulated the most dangerous level of pressure.
Centuries of heat and steam had been building beneath it.
Once the volcano truly erupted, that immense underground pressure would provide the initial force capable of hurling volcanic material far into the atmosphere.
Which meant...
"If we can realize that the current extreme cold will greatly reduce the danger caused by melting ice at the North and South Poles, then the Herrscher of Blood, who has absorbed an enormous amount of scientific knowledge, can certainly realize it too."
MEI's reasoning was straightforward.
"So there is probably another 'Temple' hidden beneath Yellowstone."
"It will be the third 'bomb,' separate from the two at the poles."
"In fact..."
"There's a good chance the bombs at the poles were never intended to explode at all."
"They may simply be decoys."
That was MEI's current conclusion.
If the Herrscher of Blood intended to destroy the natural environment humanity depended on, killing enormous numbers of ordinary people and causing the civilization of the Current Era to collapse from within...
Then MEI's answer was equally simple.
She would stop every environmental disaster Kali tried to create.
One by one.
The Newborn World Serpent might not be able to prevent Yellowstone from erupting today.
But once Prometheus's self-destruct units destroyed every Undersea Temple, Kevin and the others could still do everything in their power to freeze the volcanic ejecta during the eruption's earliest stage.
If they succeeded, they could prevent most of the ash from being carried around the planet by atmospheric currents.
Unfortunately...
"If only the Fourth Divine Key from the Current Era had been brought here."
MEI sighed.
"Whether it was the Ice Calamity or the eruption of Yellowstone, repairing the damage to Earth's environment would have been much easier."
Naturally, she knew that Prometheus Prime, who had recovered the Void Archives, could use its Mimicry ability to temporarily reproduce some of the Fourth Divine Key's functions.
But a copy was still only a copy.
Void Archives could imitate a Divine Key.
It could not reproduce the original's full performance.
In the end...
"We've done everything we can."
Clad in the Key of Blankness, MEI lowered her gaze toward the blue gem embedded in the rotating conversion mechanism on her chest.
The Herrscher of Reason's Core.
For the moment, it remained inactive.
MEI sighed inwardly.
If we can kill even one Herrscher, we'll buy ourselves enough time.
And after that...
It will finally be time for the Herrscher of Reason's Authority of understanding and reconstruction to show what it can really do.
Then...
Whoosh! Whoosh! Whoosh! Whoosh!
Four spatial gates opened.
Kevin.
Elysia.
Sim Kevin.
Sim Elysia.
All four emerged beside MEI.
They had barely assembled.
Kevin had not even had time to ask about MEI's condition before she immediately opened another portal and transported the entire group to Yellowstone National Park in North America.
At that exact moment...
"All units are in position!"
Prometheus Prime's report came through the communication channel.
MEI's expression hardened.
"Good."
"Begin."
"Yes!" ×3000+
Across the world, more than three thousand Girl Dolls simultaneously initiated their self-destruct sequences.
[Ten.]
[Nine.]
[Eight.]
[Seven...]
For a moment, everyone held their breath.
They were all waiting for the result.
Not only MEI, Kevin, and the others.
Elsewhere...
The Herrscher of Blood, Kali.
And the Herrscher of Thunder.
Both sides smiled.
But what the Herrschers were waiting for...
Was the aftermath.
[Three.]
[Two...]
"Who said there couldn't be another decoy hidden behind the decoys?"
Kali suddenly spoke.
"And the so-called 'truth' you discovered..."
"Perhaps that was simply the truth I wanted you to discover."
At the same time, the Herrscher of Thunder narrowed her eyes.
"We've won, humans." ×2
[ONE!]
BOOM! ×3333
With Prometheus Prime, a Pathstrider of Erudition, acting as the central communication hub, every Girl Doll participating in the operation abandoned its temporary Omnic body and detonated almost simultaneously.
For an instant...
The entire planet seemed to tremble.
Yellowstone.
Near the sealed caldera...
"..."
Kevin, Elysia, and their Sims stood ready at their assigned positions.
All four frowned.
Nothing happened.
The eruption they had expected...
Did not come.
No.
That was wrong.
Rattle...
Elysia's ears twitched.
A piece of rubble near her foot had moved.
Only slightly.
But she noticed it immediately.
"It's starting!"
Her voice sounded over the comms.
The next moment...
Rattle, rattle, rattle...
The trembling beneath the ground rapidly intensified.
The four combatants, positioned around different sides of the Yellowstone Caldera, immediately activated their ice-based abilities.
MEI and Prometheus Prime had already simulated this exact scenario beforehand.
According to their calculations, even Kevin, as a Returnee, would only barely be capable of freezing all the volcanic ash ejected from Yellowstone if he pushed himself to his absolute limit.
And afterward...
He would be completely exhausted.
A MANTIS, after all, could not continuously replenish energy from Imaginary Space by using a Herrscher Core as a "key" the way a living Herrscher could.
In simpler terms...
No matter how enormous Kevin's "mana pool" was, his "mana regeneration" could never match that of a true Herrscher.
Otherwise, back in the Current Era, immediately after escaping the Sea of Quanta and returning to the Proper World, Kevin would never have needed to consume Chiyou in order to restore his strength.
That was one reason MEI had sent three additional combatants to support him.
There was another.
"The Herrscher of Thunder is still an enormous threat."
MEI had already considered the possibility while simulating Kali's thought process.
"With that absurd speed, she could easily exploit Kevin's exhaustion afterward."
Both personally and strategically, MEI could not allow that.
She refused to stand by and watch Kevin place himself in unnecessary danger.
Nor could the Newborn World Serpent afford to lose one of its most important combat assets.
That was why Sim Kevin, Elysia, and Sim Elysia were here.
To divide the burden.
To preserve Kevin's strength.
And, if necessary, to protect him afterward.
Then...
BOOM!
The moment all 3,333 Girl Dolls detonated and destroyed the Undersea Temples...
Nothing happened at the North Pole.
Nothing happened at the South Pole.
There were no thermobaric detonations.
Instead...
Exactly as MEI had predicted...
Yellowstone erupted.
The ground above the concealed caldera bulged violently.
Then it split apart.
Deep underground, centuries of accumulated pressure finally broke free.
The earth ruptured.
Yellowstone burst open.
A colossal column of ash, gas, shattered rock, and burning debris exploded upward toward the sky.
But MEI had already prepared for this moment.
"Now!"
Four torrents of extreme cold erupted simultaneously.
They converged upon the towering volcanic column.
Kaka-ka!
MEI had calculated the precise moment Kevin and the others needed to act.
Too early would fail.
Too late would also fail.
If they froze the eruption before enough underground pressure had escaped, the enormous force accumulated beneath Yellowstone over centuries would remain trapped below.
That would only create another disaster.
They needed to allow the eruption to release enough pressure...
And then freeze it.
Fortunately, Kevin and the others executed the plan perfectly.
They seized the exact opening MEI had calculated.
Kaka-ka!
Ice spread upward at terrifying speed.
The enormous column of volcanic ash froze solid.
Higher.
Higher still.
Until a vast frozen spire pierced into the sky.
From a distance, it resembled a Tower of Babel sculpted by some impossible god.
They had succeeded.
After all, Kevin's ice had once demonstrated the absurd ability to freeze even the flames released by the Judgement of Shamash at the very moment they erupted.
Ice freezing fire was already ridiculous by conventional physics.
But that was precisely the unreasonable nature of Honkai Energy.
And now...
Kevin and the others were using that same absurd power to fight against nature itself.
Nature that had been deliberately turned into a weapon by a Herrscher.
For several seconds...
Everything seemed stable.
Then...
Crack.
"..."
The enormous ice pillar began to fracture.
Of course it did.
A frozen structure hundreds of kilometers tall had not exactly been designed by professional structural engineers.
Volcanic ash had already begun dispersing near its upper reaches before the freeze fully took hold.
The entire structure was top-heavy, uneven, and catastrophically unstable.
A strong enough disturbance could bring the whole thing down.
Now it was Kevin and the others' responsibility to clean up the aftermath.
Still...
They had succeeded in stopping another trigger for an extinction-level catastrophe.
At least for now.
"Sigh~"
Far away, inside a cave somewhere in South America...
Big Pardo sat inside her little tent and sighed helplessly.
She had not interfered with what happened at Yellowstone.
She could not.
But after sensing the entire sequence of events...
"If I were allowed to help, I'd just open one gigantic portal over the crater."
Pardo puffed out her cheeks.
"Wouldn't that be way easier?"
Indeed.
Her proposed solution lacked finesse.
But it was undeniably direct.
Then again, with the power of an Emanator at her disposal, even though Pardo was nowhere near the strongest among Emanators, she already possessed enough strength to completely overturn many of the crises currently unfolding across Earth.
If she were free to intervene...
Resolving humanity's environmental disasters would not be especially difficult for her.
But right now...
She was only an "onlooker."
So...
"Do your best, Boss Kevin."
Pardo's expression became slightly worried.
Her senses had become vastly more powerful after becoming an Emanator, allowing her to perceive far more than before.
And because of that...
She knew the situation was nowhere near over.
"The Herrscher of Blood's plan to destroy the world..."
Pardo muttered quietly.
"She still has plenty more tricks waiting."