Strange use of Magic (Black Clover AU)

[Third Person's PoV]

"Eh?" Asta was taken aback by Stephen's words and looked at him in confusion. "What do you mean by that?"

"AM I GOING TO DIE?!" Asta shouted, jumping to his feet and looking at Stephen with worry.

"No… It's just… you don't possess a single ounce of mana. It's literally zero. Asta, everyone in this world has at least a trace of mana, even if it's minuscule, but you don't. It's like the mana was sucked right out of you…"

Asta's shoulders slumped. "I had a feeling that was the case, but it still hurts hearing you confirm it," he said, rubbing the back of his head.

Asta then grew confused. "If I don't have mana, why am I able to use this?" He asked as he grabbed his grimoire.

Stephen placed his hands on Asta's shoulders and began to shake him. "Don't you get it? It's because you don't have any mana that the grimoire chose you. Asta, as far as I'm aware, you're the only one in the entire world capable of wielding it."

Asta looked at his grimoire, surprised. "So, my ability to cut–"

"It's not cutting," Stephen interrupted.

Asta tilted his head in confusion. Stephen went on to explain, "Asta, what you possess isn't magic… it's Anti-Magic. You don't cut magic, you nullify it completely."

"THAT IS SERIOUSLY OVERPOWERED!" Asta screamed, holding his grimoire excitedly.

"However, there's a problem…"

"What? What is it?" Asta asked, apprehensive.

"The source of your power… and what's inside that five-leaf grimoire of yours is a devil. Even though you don't have any mana, what you're technically using is Devil Magic, and you're the only one capable of wielding it. It's not wrong to say that the devil chose you," Stephen explained.

"It chose me…" Asta muttered as he held up his grimoire and smiled. "Thank you for lending me your power…"

"That's…" Stephen hesitated, wanting to warn Asta not to trust it. But seeing Asta's trusting nature, he doubted his warning would matter. Left with no choice, he continued, "Asta, we only have three weeks until the Magic Knight exams, so I'm going to train you on how to properly control and wield your Anti-Magic, so you don't get consumed by the devil…"

Asta nodded with an excited expression. "I'M READY TO LEARN!"

Stephen grinned widely. "The first lesson…" He lunged forward and thrust his palm onto Asta's chest.

To Asta's surprise, he was sent flying back, but he could see his body slowly moving. A look of horror crossed his face as he saw his ghostly hands and body.

Stephen let out a satisfied sigh. "I've always wanted to do that…" he muttered with a growing grin.

"YOU KILLED ME! I'M DEAD! STEPHEN, WHY?! I THOUGHT WE WERE BROTHERS!" Asta cried, staring at his ghostly hands and looking at Stephen with a betrayed expression.

"You're not dead, Asta," Stephen scoffed. "What you are right now is your astral body, your soul, so to speak. I've separated it from your physical body. You can return whenever you feel like it."

"Oh… I didn't really understand, but it's good to see that I'm not dead…" Asta nodded in relief. "So, why am I a ghost again?"

"You're not a ghost— You know what, forget it. What I was saying earlier, your first lesson is about sensing that hidden power within you. It's harder to feel in your physical body, but your spirit is much more sensitive and attuned to the energy around it. Although your astral body and physical body are separated, they're still connected. So, I want you to close your eyes and focus on your physical body," Stephen instructed carefully.

Asta followed Stephen's advice, closing his eyes and focusing. His brows knitted in concentration. "I think I feel something… I don't know what it is… Is it that Anti-Magic you were talking about?"

"What you're feeling is the connection to your grimoire and the devil within it. Now, listen closely. Slowly and softly tap into that connection, and like using a straw, gradually draw from it. But not too much! If you take more than your body can handle, the devil might take over. So, control it carefully," Stephen continued.

Asta slowly began to extract Anti-Magic energy from his connection to the grimoire. Stephen noticed a black line appear from Asta's chin, extending past his left eye, accompanied by a small snaggletooth on the same side.

"This is the most I can manage," Asta grunted. "I feel like if I pull any more, I'll lose control."

"Good, very good actually… You can let go now," Stephen said. Asta watched as the markings receded, and his shoulders slumped.

Stephen walked up to Asta's physical body, placed a hand on his chest, and made a pulling motion. Asta's astral body was drawn back into his physical form, and he woke up with a gasp.

"Stephen, that was terrifying! I really thought you killed me, man. Please warn me next time you plan to do that," Asta pleaded.

"No promises," Stephen chuckled darkly. "Your next lesson is to do what you just did but with your physical body this time. You should now be more familiar with your energy due to your experience with your Astral body, though it will be much harder."

Asta nodded and sat cross-legged on the ground. He focused on drawing from his connection to the Anti-Magic energy, but it was more difficult than he had expected. It wasn't until his fifth try that he succeeded.

When he managed it, he opened his eyes, which had shifted from green to glowing bright red. His expression showed he was struggling slightly.

"Even though I feel stronger, I have to concentrate so I don't lose control. I'll need to learn how to fight and keep my focus at the same time if I want to use this power effectively."

Stephen nodded. "That's why we're training. You'll learn to manage it, and it'll benefit me too—I'll finally have someone to train my martial arts with."

Eldritch magic circles formed around Stephen's forearms as he threw a punch toward Asta. Asta's instincts kicked in, and he leapt out of the way, landing on his feet as the boulder he'd been sitting on shattered, sending rocks flying everywhere.

"THAT WASN'T FAIR! I WASN'T READY!!" Asta shouted.

Stephen glanced at his arm, flexing his hand and studying the magic circles that had enhanced his strength. He flicked his wrist and smirked. "Cry about it," he taunted, before flying to Asta. The Cloak of Levitation billowed behind him as his forearm, encased in three glowing magic circles, increased his strength threefold.

Thus, Stephen continued to spar with both Asta and Yuno during the day, slowly helping Asta get a handle on Anti-Magic. At night, Stephen trained his time magic, carefully practicing within his grimoire. Though it wasn't his strongest set of spells yet, he was becoming more proficient, but unlike with his other magic, he treated time magic with great care and caution.

And then came the day for Magic Knight Exams. 

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