Rune Magic.
It is the way humans harness mana that is hanging in the world. They have been doing this since before any living being could manipulate mana inside their body, as it was recorded in the history text that said rune magic is the way the human race could survive against demons and elven races in the ancient time, considering both races have more advantageous than human.
However, when humans started discovering how to manipulate mana inside their bodies, they left the rune magic slowly, as they saw it as an ineffective way to use the mana. And they are not wrong, considering how long it takes to create just one rune.
For example, if you want to create fire, you need to carve the word to the solid object with a feather from Ena, a rare creature that resembles Pegasus from the earth. However, they cannot create something out of nothing. That is why they cannot just carve 'fire' to ignite a fire out of nothing.
No, that is not how rune works.
They need a source to fuel the rune to make it work. That is why an air-based rune is the most popular among the practicians, second to earth and water-based element. Because those three are the most common thing that could be found anywhere. So, if one wanted to ignite fire out of nothing with a rune, the words they need to carve are 'air friction' or something like that.
To put it simply, rune magic follows the similar logic as how programming work in Earth.
That is why rune magic is unpopular after humans discovered how to manipulate mana inside their bodies. Now, with only a single hand movement, they could ignite fire out of nothing in a second, way faster than rune. And the best part is it doesn't need to be studied for anyone to use it, unlike runes where you need to know what is caused what to create something.
However, as living being evolved, they also found a better use for runes.
The first time humans found out about another usage of the rune is in the first war between humans and elves when humans couldn't able to keep up with elves in the night battle. One of the mages at that time proposed the idea of using runes to detect the enemy's movement in the night so they don't have to stay on guard all the time.
And that is the first usage of runes that becomes common nowadays, as a detection and the first defensive layer to prevent enemies from entering private areas.
However, even though the usage of rune magic is crucial, a lot of people still look down on it and think that it is useless. Argo couldn't help but snort in a mocking manner every time he heard the debate between nobles in the Musia Kingdom about how useless Rune is and how they should cut its research budget.
'Those greedy bastards only care about their money. If someday there is an attack from the other kingdoms, they would beg for any defensive measurement for their lives.'
"Big brother, are you okay?" A light tap on his shoulder snapped Argo from his thought. When he turned around, he could see Vera looking at him with a worried expression. "You didn't respond when I called you. Should I call big sister?" And in that time Argo just remembered he went to his room to pick up the tool for the rune.
Argo let out a chuckle as he ruffled Vera's hair.
"Thank you for worrying me, Vera. But it is alright." He smiles softly. "Let's go back to the other. They must be waiting for us."
"Um!" The girl nodded rapidly, showing enthusiasm for what Argo would show to them. When they both left the room, Argo could see the similar expression on the children's faces, eager to see what the other cool things that they could learn in the future.
"Well, because we need to go outside, let's wear this first." Argo summoned a pack of masks from his storage space and shared it with the others.
Honestly, he feels stupid for bringing a lot of masks when he left the Earth. But it is better safe than sorry, considering the condition on Earth when he left was just hit by a massive pandemic. And fortunately enough, it is quite useful at a time like this.
"Okay, everyone is ready?" Seeing the nod from the children who have already worn their masks, Argo opened the door and they were welcomed by a very dark sight.
"Woah! It is very dark here! I cannot see anything!" Alex exclaimed in amazement. Being children, they got overly excited by the fact that the city where they lived suddenly turned into something that could be found in a black-and-white movie.
"Alex! Deron! Don't go too far and always wear your mask!" Argo yelled at two siblings who run immediately after they are out of the building. He only sighed when he was ignored by the siblings and turned around at Gavin.
"Gavin, can you check out those two and the condition of the city?"
"Yes, My Lord!"
Right after that, the demon emerged into the shadow, leaving Argo with Vanda, Ara, Vera, and Lami in silence.
"Well.." Argo spoke, breaking the silence. "Let's get to work, okay?" And right after that, he bought out the feather that he picked up from his room and showed it to the children. The three of them looked at the majestic feather in awe, never seeing something as beautiful as it is before.
"Big brother, what is that?" Vera asked, her mouth still gaping wide.
"This.." Argo gave one of the feathers to the girl. "Is a feather from Ena, a majestic creature that resembles a horse with a pair of wings on its body. And this is our tool to create rune magic to cleanse this city."
The children are still looking at the feather in awe. If just by looking at the feather they could feel how majestic this creature is, they cannot imagine how magnificent it is to see the creature itself.
"But big brother…" Lami chimed in hesitantly. "Is this okay? I mean, I heard from the kingdom that Ena is extinct, and it is forbidden to use any parts of its body for any purpose.."
Hearing that, Argo snapped his head hard to Vanda, silently asking for confirmation. Seeing the slight nod from the elf, Argo pondered for a moment before he sighed.
'Well, I should expect this. Fifty years without contact of course would be resulted in me not knowing a lot here. I will ask Gavin about it later.'
After pondering for a moment, he shakes his head and smiles before he patting Lami's head.
"Don't worry about it, Lami. While people might think Ena is extinct, that is not true. The existence of the feather in Vera's hand is enough evidence for me to believe it."
"What do you mean, big brother?" Now, not only the children, but Vanda also has her body leaned forward in curiosity.
"Well, this is only something that only people who studied about creatures would know. If Ena died, everything about it from head to nail would be burned to prevent anyone use parts of their body for anything. That is why if this feather still here, the Ena who gave it to me is also still alive here."
"Really?"
"Yup!" Argo nodded. "I don't know why people think it is extinct. Maybe because they decided to hide after they keep being hunted almost all the time, or maybe because some greedy people think they could keep it for themselves. But I will ask Gavin to keep his eyes on it, okay?"
"Um!"
"Okay, so.. Let's start with this!"
And right after that, Argo stomped the ground, making it raise about his hips.
"Woah! Is this your magic, big brother?!" Both Alex and Gavin who just finished running around like any normal children suddenly pop up in front of him, looking at his magic with shiny eyes.
"I didn't even hear your incantation! That is amazing!"
"Yeah! Can you teach us how to do that?"
Argo chuckled at the boys. "Sure. We can do that in the future. But for now, just watch me, okay?" Seeing the boys nod, he took a deep breath before his hand is starting to dance on the ground that he had just raised. He used it as the medium of the rune that he is about to make.
"Woah.. Big brother's handwriting is pretty!" Lami exclaimed, probably the only one who noticed that. Hearing what Lami said, Vera pouted slightly at no one.
"I don't know how to write.." She suddenly muttered, causing an awkward silence around them. Even though Argo is still concentrating on his work, he can see a pang of guilt on Ara's face. Probably cursing himself for not being able to provide the necessary education for Vera, something that he would need to address later.
"Don't worry about it," Vanda replied to the girl. "I can teach you how to do that later if you want."
"Really?"
"Yup! It is a promise."
The girl's eyes lightened up once again as she launched herself into the elf, hugging her happily.
"Thank you, big sister! You are the best!"
Right after that, Argo decided to put his focus back on the work in front of him.
"Hmmm.. I don't know a lot about carbon monoxide, but I know there is many ways to capture carbon dioxide in the air. It would be easier to do that rather than trial and error. So, first, I need oxygen to make it react and become carbon dioxide."
Then he turned around and spoke to the others.
"I will make winds! Brace yourself!"
A second later, a strong wind is blowing in the air, threatening to destroy anything it passed.
"Kyaaaaa!"
The children are frightened at the sudden change in the air. Fortunately, Vanda acted quickly and formed a hand seal to cover the children.
[Vine Magic: Shield]
Suddenly, vines grow from the ground, creating a huge dome that protects them from the wind. Even though they could still feel the wind, now they are well protected inside the vines dome.
"Haah…" Vera sighed in a relieved tone. "Thank you, big sister! You are the best!"
Argo ignored the children for now and put his focus on the work in front of him.
"Hmm.. I don't know how much oxygen could be produced with that. Fortunately, it is purely from nature and has no mana involved, so the carbon monoxide wouldn't be multiplied again. But for now, let's make a simple net to catch the carbon dioxide."
Even though he knows a method called Direct Air Capture, he didn't know a lot of the detail about that. So, he decided to use the most traditional way to cleanse this city. Tree planting.
"Vanda!" He called the elf. "Help me write this sequence!" Vanda comes over to see his work, followed by the children who looked at him curiously. "Your affinity toward plant magic is stronger than mine, so I need you to write the rune line to make at least this area surrounded by a little forest. Can you do it?"
Vanda frowned for a moment before she asked a question.
"Sure, I can do it. But why not use my magic? It would be easier and faster, right?"
Argo just shook his head at her question.
"No, that wouldn't do. Just like I said, the exposure of mana would make thing worse. I have already told you about it. But with rune, you will just trigger the nature to act like what you wanted without involving mana. This is my best solution."
Vanda pondered for a moment before she nodded.
"Even though I don't know what you are going to do, I will trust you for this." She rolled her sleeve before she began to work. And a minute later, the line to grow up the little forest as Argo called it has already finished. When he saw her works, he nodded din satisfaction.
"Trees grow within 500 meters radius with at least two meters distance between each other. Good. Now, I will add this as the final touch."
He added the sequence of the forbidden life magic from the elven race there, causing a gasp from Vanda.
"You know about it too, big brother?!"
Argo snorted.
"Of course, I do. Why do you think your elders hate me so much in the first place?"
And with that, he left Vanda speechless at the shocking revelation.
"Hmm… Okay, this sequence will make this forest absorb anything they consider as energy in this area, which would be the carbon dioxide. And the life magic would act as the sunlight here. But with life magic, the growth of the trees would be uncontrollable. I need to be careful about this."
After writing the rune carefully, he finally sighed in relief as the rune began to glow. And within a second,
*GROOOWLL!!*
A lot of small trees suddenly grow from the ground, completely shocking the children.
"Woaah! Is this what you are doing, big brother?!"
"Yeah…"
"But you said it is to cleanse the city. How can a lot of trees do that?"
Argo couldn't help but smirk at Ara's question.
'As expected, he is the sharpest and the most mature among the children. Let's see if his brain could be interpreted as his fighting skill too.'
He shook his head, snapping him from his thought.
"Well, you will see about it tomorrow. Let's go back to the house first."
He turned around and go back to his home, followed by Vanda and the children.