When I Tried Strengthening 【Rusted Sword】, It Evolved Into An Overpowered Magic Sword

Chapter 75 Proof of strength

Since I arrived here, it’s been just under an hour.

I had been running at full speed, so I thought I had put quite a distance between myself and Dia. Yet, she arrived much faster than I expected.

She must have rushed here with all her might, as evident from the sweat on her brow and her labored breathing.

“Y-you blocked my poison magic…? Unscathed… with that shield…?”

Dia’s sudden appearance in a crucial moment left Black Snake visibly bewildered. It was a natural reaction; after all, a slender girl had taken on an attack that should have been avoided head-on.

Similarly, Onyx, who was locked in a blade lock with me, showed surprise.

“Hey, hey, another weird kid shows up. Are you another adventurer who was with this guy?”

“…”

I responded with a silent nod. Black Snake clenched her teeth, clearly struggling to believe what was happening. Then she shouted at Dia.

“Who are you!?”

Dia, with a tone similar to how she was addressed, replied to Black Snake.

“I’m the adventurer Dia Carat. I came to help my comrades. And I won’t let you harm the fallen.”

For once, Dia directed a sharp gaze at the demon. The usually reliable Dia appeared even more dependable now. The mere presence of her brought such a sense of relief.

As I was reminded of this, Black Snake narrowed her red eyes and glared at Dia.

“Won’t let me harm them, you say? Just because you happened to block one of my spells, don’t get too full of yourself!”

With a shout, she pointed the mouth of the pot she was holding directly at Dia.

“【Corrosive Venom Melt】!”

In an instant, the poison liquid from the pot was shot out.

This time, it wasn’t just one attack. Black Snake chanted the spell name repeatedly, unleashing a barrage of poison magic.

With many adventurers behind Dia, there was no option to evade. It was a merciless attack, fully aware of the situation. Yet Dia showed no sign of fear and positioned her large white shield in front of her.

With a series of dull thuds, all the attacks were blocked by the shield.

Black Snake continued her assault while cursing.

“Ugh, move out of the way!”

“I absolutely… will not move…!”

Dia blocked all the poison magic with her shield to prevent harm to the adventurers. Seeing this from the side, I couldn’t help but sigh in relief.

Without Dia, I would have had to watch those people die.

Thanks to her, I avoided showing Citrine a despairing outcome.

Noticing my relief on my face, Onyx in front of me let out a dry laugh.

“Hah! What’s with that face? You think you’re so tough just because a kid showed up? Let me tell you, nothing has changed. All that’s happened is that one more corpse is added.”

I glared at Onyx, trying to appear intimidating. However, he was right; the situation hadn’t changed much. Although Dia’s arrival slightly reduced the risk of taking hostages, the core issue remained that we needed to defeat Onyx and his group to resolve the situation.

And I was still struggling to take him down.

The fact that the blade lock remained balanced was proof enough.

As I clenched my lips in silence, Onyx continued to wear a smug grin.

“Besides, I still wanted you to keep me company. This turn of events is actually quite convenient.”

“…?”

Convenient?

Before I could question the meaning of his words, Onyx suddenly lifted his leg.

By the time I realized it, I had already been kicked in the abdomen.

“Ugh—!”

Fast. And heavy. His legs were incredibly strong.

If not for the increased resilience granted by the divine artifact, my entire body would have been shattered in a single blow.

The impact of the kick sent me crashing into the rock wall behind me, hitting my back with a jarring force.

Momentarily stunned by the pain, I was caught off guard as Onyx sheathed his sword in his right hip and suddenly lunged for Citrine.

“—!?”

Citrine tried to escape, but was grabbed by Onyx without a chance to cry out.

Was he planning to take Citrine hostage? Why now…

Confused by his intentions, I watched as Onyx carried Citrine under his left arm and began running toward the exit.

The first to speak to Onyx’s back was Black Snake, his companion.

“Wait, Onyx-sama! Acting on your own is—!”

“If you want her back, come after me, Rust Stone! Black Snake, I’m leaving that silver-haired girl to you!”

It was only then that I realized Onyx’s plan.

He intended to separate Dia and me. By doing so, he aimed to disperse our fighting power and create a one-on-one situation.

Moreover, he could keep Black Snake here as a watchdog.

This was bad. If I didn’t hurry, I’d lose sight of Citrine.

But, right now, Dia and the injured adventurers were here.

I couldn’t leave them behind…

“Go, Rust-san!”

“…Dia”

“Leave this to me! I’ll make sure to protect everyone!”

So go quickly—! Dia implored with her gaze.

After a moment of hesitation, I nodded silently and started running.

Even knowing this was likely a trap, I couldn’t afford not to pursue him.

If I lost Citrine here, it would be irreparable.

At a glance, the interior of Obsidian Mountain was a maze of narrow passages.

Each passage led to a large chamber, and viewed from above, it resembled an ant’s nest.

If Citrine were hidden somewhere, I might lose track of her.

In the worst case, she could be taken and escaped with.

Worried about this, I chased Onyx’s back with determination.

“Damn it—!”

The distance that barely closed brought a sense of urgency.

Even though he was partially equipped and had put away his divine artifact, he was remarkably agile.

I was gradually losing track of my location.

Or rather, Onyx was deliberately choosing paths to confuse me.

Separated from the comrades I had just joined, the frustration was mounting as I finally reached a large room.

It was slightly smaller than the dome-shaped space earlier but still a vast chamber.

The clear difference was the number of lights, making visibility better. Was this someone’s room?

“With this, no more intruders. Let’s settle this, Rust Stone.”

“…”

Onyx threw Citrine to the ground and drew his sword again.

I readied my right hand’s 【Cursed Magic Sword】.

Being separated from Dia was problematic.

We couldn’t help each other in a pinch.

Dia lacked a way to defeat demons, and I wasn’t sure how long I could use the 【Cursed Magic Sword】.

Moreover, Onyx’s sword. The moment it was enchanted with magic, its aura clearly changed.

Ideally, Dia would defend against it with her 【Immortal Great Shield】 while I attacked Onyx during the gap.

But Onyx, likely out of a desire to avoid interference, had unintentionally prevented our cooperation.

I had to defeat him quickly and return to Dia.

So, I was about to strike, but first, I decided to clear one doubt.

“Why go so far to have a one-on-one fight with me?”

“Eh?”

There were many aspects of Onyx’s behavior that I didn’t understand.

He had gone out of his way to persuade his comrades to have a one-on-one fight with me and hadn’t taken any hostages.

Now, he even went so far as to kidnap Citrine to make it a two-person fight.

It seemed inefficient. What exactly was he trying to accomplish?

I voiced this lingering question, and Onyx responded with what seemed like a hint of exasperation.

“I told you, didn’t I? It’s to have some fun. Fighting is the most enjoyable game. I’m having the time of my life with this game. So I won’t let any interferences come in.”

“…”

Fighting as a fun game.

So, he wanted to be one-on-one with me just to enjoy the game to the fullest?

It was clearly beyond my comprehension.

I thought he might be hiding some rational reason, but it seemed there was none.

“Also, when you lose and start making excuses, it’s a letdown. So, when facing strong opponents, I usually fight one-on-one.”

“…A strong opponent?”

“Yeah, that’s right. You’re strong. I’ll give you that. You’ve got the same or even greater strength than the guy who cut off my right arm. That’s why I wanted a one-on-one fight with you.”

“Cut off your right arm?”

Who was that person?

As I pondered this question, Onyx spoke as if reminiscing about a distant past.

“That guy who cut off my arm was strong too. He was an adventurer. I wanted to kill him with my own hands, but he died of some pointless illness years ago. It really felt like he got away with a win. That’s why I’m fighting to prove my own strength. I’ll show everyone that I’m the strongest as the Demon King.”

To prove his strength.

Earlier, Onyx said he was enjoying the fight to the fullest because “fighting is a fun game.”

But that was probably the real reason he wanted a one-on-one with me.

At the same time, it was also his “reason for fighting.”

He seemed to have lost to an adventurer before, and since he couldn’t get a rematch with that person, he sought out other strong opponents.

He hated being left with a loss, and out of frustration, he was trying to defeat other strong opponents to prove his strength.

It seemed like mere self-satisfaction to convince himself he wasn’t weak.

For such a reason…

“As long as I can prove my strength, that’s all that matters. That’s enough reason to fight.”

Onyx, with a low voice, raised his left-hand sword high.

With a sinister grin, he charged at me, ready to strike.

“So, become proof of my strength! Rust Stone!!!”