Project Necro is an official initiative by Re:Library.

The military ship advanced while towing the sailing ship, slipping past the tentacles. Because the reefs could not be seen visually due to the fog, they needed to get a little closer.
However, if that octopus b̲a̲s̲t̲a̲r̲d̲ had enough intelligence to use the wreckage of the sailing ship as pseudo-bait, it should be getting bored of the phantom ship by now.
This time, we might be the ones attacked.

The size of the reef had been measured by the mermaids. They could sing underwater and detect their surroundings by the echoes of sound. Excellent sonar operators.

“Veit, it’s about this big.”

They formed a circle and told me the rough size.

“That’s huge…”

It’s about the size of two tennis courts.

“Rasie, how many more illusions can you deploy?”

“If it’s just creating them, I could do ten or twenty, but I can only properly move one at a time. I’d have to focus on it completely.”

Rasie answered apologetically. It seemed impossible to increase the number of decoys any further.

“Alright, you can leave the ship illusion alone now. Prepare for the next step.”
“Yes.”

With four tentacles already around here, the main body had to be close.
Sure enough, a reef soon appeared within the fog. The fog was especially dense there, so it couldn’t be seen unless you got very close.
If we hadn’t had mermaids and centaur soldiers, we might have run aground.
But now, we had grasped the full picture of this octopus b̲a̲s̲t̲a̲r̲d̲.
The part of the reef above the water was about six tatami mats in size. The rest seemed to be submerged underwater.

I pictured the octopus in my head and imagined its state beneath the surface based on the visible legs and the reef.
I had seen plenty of octopuses in aquariums and fish markets in my previous life, so I could manage the image.
Alright, it should be around there.
I ordered the dragonfire engineer unit to fire the catapult.

“Target: sea surface in front of the reef! Fire the first shot!”

The dragonfolk immediately repeated the order and operated the catapult with practiced hands.

“Target: sea surface in front of the reef! Firing the first shot!”

Garsche, who had been berating the timid sailors, saw this and let out a hopeful voice.

“Oh! So that’s the Demon Lord Army’s secret weapon!”
“Well, something like that.”

The barrel loaded into the catapult was launched with great force.
Drawing a parabolic arc, the barrel splashed down into the sea exactly as aimed.
And then it floated there.
That was all.

Garsche stared at it blankly, then hurriedly turned on me.

“H-hey! Nothing’s happening!”
“Calm down. That was an observation shot.”

We only had one real round. If we threw it into a strange place, that would be the end of it.
If this thing performed to its maximum effect, the battle would become much easier all at once.
That was why we had to be careful.

“Load the next round! Prepare to fire ‘Thunder Salt Silver’!”

A faint tension ran through the dragonfolk.
But Kurtze, the engineer, immediately repeated the command.

“Roger! Preparing to fire ‘Thunder Salt Silver’!”

What was mounted on the catapult was a barrel the same size as before.
Seeing it, Garsche asked me, unable to suppress his curiosity.

“That’s the one?”
“Yeah, this is the one.”

It was no joke of an item, so I was honestly uneasy myself.

Kurtze brought the barrel over and carefully set it on the catapult.
The octopus’s tentacles were rampaging all around, but panicking here would lead to disaster.

“Centaur soldiers and mermaids, retreat to the rear of the flagship! Use the hull as a shield!”

After confirming our allies had withdrawn, I gave the order.

“Fire ‘Thunder Salt Silver’!”

The catapult’s arm roared with a violent sound and hurled the barrel.
The barrel splashed down on the intended spot on the sea surface and floated there.

“Hey, it’s the same as before!”

Garsche shouted, but this was no time for that.

“Rasie!”
“I understand!”

Just as in training, Rasie cast an illusion on the barrel.
In an instant, the barrel transformed into a human flailing on the surface of the sea.
Or rather, it looked exactly like Rasie. Even the way it struggled was just like her when she first started swimming practice.

“Isn’t that you?”
“Yes. Easier to imagine the movements that way…”

That’s fine, but you know that’s going to get eaten by the island octopus, right?

The island octopus, which seemed to have grown thoroughly bored of fiddling with the phantom ship, did not miss the foolish young girl who had fallen right in front of it.
The fake Rasie was immediately wrapped up tightly by the octopus’s tentacles and dragged underwater.
Even knowing it was an illusion, it was a rather shocking sight…

“Hey, what’s about to happen?”

Garsche asked anxiously, and to be honest, I was worried too, so it was hard to answer.
But as the person in charge of the Demon Lord Army, I needed to act confident.

“Everything is proceeding as planned.”

The next moment, a massive explosion occurred underwater.
A pillar of water rose, and seawater rained down. Yellow light flashed repeatedly beneath the surface, and the sea boiled.
All of the writhing tentacles recoiled in surprise. The wreckage of the Belrüsa merchant ship was hurled into the sea.

“Yes!”

It worked, and I shouted without thinking.
I explained things to the stunned Garsche.

“That explodes when it comes into contact with water. A secret weapon of the Demon Lord Army.”
“What the hell is that… amazing.”
“Moreover, the water it touches turns into a deadly poison that dissolves living creatures.”
“You guys are demons.”
“We’re the Demon Lord’s Army.”

At least officially, we are the villain faction.

What I had fed to the island octopus was a barrel containing tightly sealed metallic sodium.
Back in high school, a friend from the chemistry club once showed me a video of metallic sodium exploding. When a large container of metallic sodium was thrown into water, it caused a huge explosion.
I thought that explosion power and the subsequent water contamination would deal severe damage to aquatic monsters.
I had also heard that since it was chemically unstable and reacted even with moisture in the air, it was usually stored in kerosene.
Of course, it shouldn’t naturally exist in this world either.

The ones who made it were the late Demon Lord and Master.
The late Demon Lord had apparently been experimenting to see whether his scientific knowledge would work in this world. It was written in his notes.
However, it seemed that the constituent elements of substances in this world were slightly different, so the results often differed from expectations.
So I couldn’t say for certain whether it was truly metallic sodium. Its properties were almost the same, so it probably was.
Since it produced the result I wanted, it didn’t really matter what it was.

I was satisfied with that, but Kurtze, the engineer, sighed beside me.

“To use the legacy of the late Demon Lord like that… Sir Veit, why do you insist on making everything explode?”
“That’s what military men do…”

I could feel the gazes of the engineers on me, but since this would let us fight more advantageously, I hoped they’d overlook it.
Kurtze added, as if remembering something.

“It seems to have been effective as a weapon in the end, which is good, but yellow dragon orbs will be difficult to produce for a while, so please understand that.”
“Understood. Sorry about that.”

Yellow fireworks are made using sodium compounds.
It’s the same phenomenon as when you drop salt onto a stove flame and the fire turns yellow.

“Kurtze, can’t we substitute it with salt?”
“Salt absorbs moisture, you see.”

I said I was sorry already.

I should double-check that the chemical formula is correct.
Let’s see, sodium is Na, and water is H2O, right?
When they combine, you get strongly alkaline sodium hydroxide. That would be NaOH, leaving one extra H… oh, right, the gas produced is hydrogen.
I see, that’s why it explodes.
I hadn’t thought very deeply about the mechanism behind the explosion before, but now I finally understood.
Chemistry is interesting.

The island octopus must now be writhing in agony as the inside of its mouth burns and melts away.
All of its tentacles were thrashing, smashing the surface of the water. The wreckage of the Belrüsa merchant ship took a blow from a tentacle, and its mast snapped off at the base.
It was rampaging more violently than expected.

“Don’t get caught in the tentacles. Don’t approach carelessly! And the seawater around it is deadly poison!”

I wanted to press the attack all at once, but we couldn’t get close until it weakened a bit more.
The island octopus stops the surrounding wind and currents with the magic power imbued in its body. It doesn’t consciously use magic; it just happens automatically as an innate ability.
That’s why it can’t escape by riding the current, nor can the sodium hydroxide solution be washed away by ocean flows.
Covered in a strongly alkaline solution, its body surface should be dissolving more and more.