The grassland inside the Chrono Cube stretched endlessly under the pale white sun.

A soft wind moved through the field, but unlike before, the area was calm. No bots, no explosions—just open space.

Selene stood there, wand in hand.

It had been six days since the training began.

For the first three days, she had done nothing but fight. Again and again, against the E.I.R.A bots, until her body adapted and her mind finally stopped resisting. Only when Zane was satisfied did that phase end.

The next two days were different.

No constant fighting.

Instead, Zane refined her control. He corrected her flaws in parallel thinking and dual casting—things she had been using instinctively but without full understanding. Under his guidance, what was once unstable became precise.

And now—

The sixth day.

A new lesson.

Zane stood in front of her, hands in his pockets, watching her calmly.

“Selene,” he said, his tone slightly more serious than usual. “Listen carefully.”

Selene straightened unconsciously. Her deep blue eyes were focused, but there was clear excitement in them.

“I’m going to teach you something,” Zane continued, “that will increase your strength by leaps and bounds.”

Her grip on the wand tightened slightly.

Zane smirked faintly.

“I’m going to teach you how to create a special type of rune.”

Selene leaned forward just a little, fully attentive now.

“Self-sustaining runes.”

Selene blinked slightly.

“Self-sustaining runes…?”

Zane nodded.

“Yes. Self-sustaining.”

He raised his hand casually as if explaining something simple.

“Normal runes need a constant mana supply, or you fill them like a balloon. Once the mana inside runs out, they collapse and disappear.”

Selene nodded. That was basic knowledge.

Zane continued, “But self-sustaining runes are different.”

He paused for a moment, letting it sink in.

“Once you form them… they maintain themselves by drawing ambient mana from the surroundings.”

Selene’s eyes widened.

“That means…”

Zane finished her sentence.

“You won’t need to constantly feed them mana.”

He looked at her calmly.

“You’ll only spend mana while creating them. After that, they sustain themselves for a long time.”

A brief pause.

“And more complex rune constructions become possible.”

Selene remained silent.

Processing.

Then suddenly—

Her eyes widened even more.

“Wait… isn’t that… the same as the ancient runes found in labyrinths and ruins?”

She looked at him with disbelief.

“Professor… you know ancient rune magic?”

Zane nodded without hesitation.

“That’s correct.”

Silence followed.

Selene looked at him, her expression filled with amazement.

“That’s… incredible.”

Zane chuckled softly.

“It is.”

Then his smile sharpened just a little.

“But it won’t be easy.”

Selene’s excitement didn’t fade.

If anything—

It grew stronger.

After all, she was about to learn something that the current world had already lost.

.

.

.

Zane raised his finger and slowly drew a rune in the air. The lines formed one by one, precise and steady, until the rune floated completely in front of them.

“Watch carefully…”

Selene’s gaze locked onto it without blinking.

At first glance, it looked like any normal rune. But as seconds passed, she began to notice something was off. The rune didn’t flicker. It didn’t collapse. It just… stayed there.

Time passed.

One minute… two… five…

Even after ten minutes, it was still floating, glowing steadily.

Selene narrowed her eyes slightly. There was something subtle in the air around it—a faint distortion, almost impossible to notice. The rune wasn’t being supplied by Zane.

It was pulling mana on its own.

Zane pointed at the rune. “Structure, flow, and lastly function or purpose. These three are what you need to learn to create a self-sustaining rune.”

Selene didn’t look away. She kept observing the rune while listening carefully.

Zane continued, “First—structure. How you build the rune. There are millions, even billions of possible structures. But you don’t need all of them. In future you will many of them on your own.”

A sheet of paper appeared in his hand, and he handed it to her.

“I’ve listed thirty-three optimal ones for beginner like you.”

Selene shifted her gaze to the paper, scanning it quickly.

Zane went on, “They are divided into three categories based on mana capacity and output. Eleven low capacity, eleven medium to high, and eleven very high.”

Selene nodded slowly, her eyes moving between the paper and the rune again.

“…This one,” she murmured, “third from the low capacity section…”

Zane smirked slightly. “Good.”

Then he said, “Now look here.”

Selene lifted her gaze toward him.

“Second is flow.”

Zane continued.

“Structure of these runes possess a intake for mana”

“And this is very important… Aside from structure, flow also controls the stability of the rune…”

Zane raised both hands at the same time, and two runes appeared in the air. Their structure was identical, but the difference was obvious.

One was glowing intensely, almost overflowing with energy.

The other was dim, barely holding its form.

They lasted only a moment.

The bright one suddenly burst with a small popping explosion and vanished, while the dim one simply crumbled and disappeared at the same time.

Zane looked at her. “Did you understand?”

Selene nodded immediately.

“Yes… their flow was not good. One had too much mana , the other too little.”

Zane nodded. “Correct. It all depends on the intake.”

He continued calmly, “If the intake is too wide, the rune absorbs too much mana and overflows, causing it to explode. If it’s too narrow, it won’t receive enough mana to sustain itself, and it collapses.”

Selene looked back at the empty space where the runes had been.

A small smile appeared on her face as she processed it.

“…Wow.”

Zane looked at the floating rune for a moment, then said, “Finally… the last one. Function.”

He pointed at the rune he had created earlier.

“Selene, did you notice anything different about this rune?”

Selene looked at it carefully this time. Not just the structure… not just the flow… but the whole thing.

After a few seconds, she nodded.

“…It’s not doing anything.”

Zane smiled.

“Exactly.”

He folded his arms slightly.

“You now understand structure and flow. But if you don’t define what the rune is supposed to do… then it’s useless.”

A small pause.

“That’s why function exists.”

He waved his hand, and the rune disappeared.

Then he created two new runes in front of her.

“Now look at these.”

Both runes had the same structure as the previous one.

At the next moment—

They activated.

Two wind blade slashes shot out from them simultaneously.

Selene’s eyes widened slightly, both amazed and surprised.

The runes continued.

Another slash.

Then another.

But after two or three attacks—

One of them dimmed, flickered, and crumbled into nothing.

The other—

Continued.

Another wind blade formed and launched.

Then another.

Zane glanced at her.

“Do you know why that happened?”

Selene frowned slightly, her eyes still on the runes.

Same structure.

Same function.

But different result.

“…I don’t understand…”

Zane nodded.

“It comes down to flow again.”

He pointed at where the first rune vanished.

“That one consumed more mana than it could absorb.”

Then at the second one.

“This one maintains balance.”

He continued calmly,

“When you create a self-sustaining rune, you must calculate two things—mana consumption for its function… and mana required to sustain its existence.”

A brief pause.

“The intake must be higher than both combined.”

Selene listened carefully, her expression slowly shifting into understanding.

Zane looked at her.

“Understand?”

She nodded.

This time, clearly.

Zane smirked.

“Fantastic.”

He snapped his fingers, and several rocks appeared in front of her, floating slightly before dropping onto the ground.

“You can create normal runes in the air easily. But self-sustaining ones are more difficult.”

He gestured toward the stones.

“So you’ll practice on these first.”

“Once you stabilize them… we move to the next step.”

Selene looked at the rocks.

Then at her wand.

Then back at Zane.

A small smile formed.

If I combine multiple self-sustaining runes…

The thought started forming clearly.

I can create arrays… automatic attacks… defensive triggers… healing activation…

Her eyes lit up.

Ideas began flowing one after another.

She tightened her grip on the wand.

“…This is amazing.”

Then she stepped forward.

No hesitation.

And began practicing.

From a short distance, Zane watched her.

A faint smirk appeared on his face.

“She’s thinking in the right direction…”

—

On the other side of the grasslands.

Drevin stood still.

Or rather… half-still.

His body remained upright, but his lower half was slowly sinking into his own shadow. It wasn’t normal darkness—it moved, rippled, as if alive, pulling him in little by little.

His breathing was steady, but his eyes were focused.

Zane stood in front of him, watching carefully.

“Good,” he said calmly. “You’re doing it.”

Drevin didn’t respond.

His focus didn’t break.

The shadow around him deepened, spreading slightly, responding to him instead of just existing beneath him.

Zane nodded faintly.

“Keep it up.”

—

Far from there—

A distant wolf howl echoed across the grassland.

—

All three of Zane’s student were training.

Pushing forward in their own ways.

Zane stood alone now, his gaze sweeping across the entire space.

Everything was moving according to plan.

A faint smirk appeared on his face.

“…Good.”

Then, almost to himself—

“This is going well.”

A small pause.

“And it’s fun too.”

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