Reincarnated in DXD(rewrite)

Gremory Territory, Training Arena

Over the past several months, Maxius had fully embraced his training under the watchful eyes of his teachers, his mind and body adapting to the magical and demonic abilities that came with his new life in the Gremory household. Alongside his studies in the intricate world of magic, he had also taken on the task of understanding the many forces at play in this realm. His thirst for knowledge had already pushed him far beyond the limits of a typical four-year-old child. However, he understood that theory was not enough. Practical experience was crucial, and so, on this day, he decided to push his limits even further.

In the massive training arena, four figures stood in the expansive space, preparing for what would surely be an intense and educational spar. Maxius, still a child by appearance, stood with a determined glint in his blue eyes. His bright red jumpsuit bore the unmistakable Gremory circle on his back, while his black and red sneakers were perfect for swift, decisive movements. Around him were three adult guards, fully armored, with training weapons in hand, ready to provide a challenge.

"Alright, are you guys ready?" I called out, my voice steady as I focused on the task at hand.

"Yes, young master!" came the synchronized reply from the three guards, their expressions a mix of excitement and professionalism.

"Then let's get started!" I shouted, raising my left hand and gathering high-density mana. A small but potent bolt of lightning formed at the tip of my index finger. Without hesitation, I unleashed a concentrated surge of power toward the guards.

Byakurai!

The bolt of lightning shot through the air with a piercing crackle, and the guards immediately moved into a defensive formation. One of them raised a magic circle barrier just in time to absorb the brunt of the attack. However, the sheer force of my magic caused the barrier to flicker, crack, and tremble. Even though they reinforced the barrier with more mana, the intense heat from the lightning attack began to scorch the earth beneath them, causing the ground around them to blacken.

The energy finally dissipated, but the aftereffects were still evident—the air was thick with smoke, and the guards stood, drenched in sweat, their faces reflecting both shock and respect. For them, it was a sign of how far I had come since we first started training. But when they turned to me, they saw a different sight.

I was exhausted. My small body was trembling from the strain of the spell, sweat dripping down my brow as my breath came in shallow gasps.

"Damn," I muttered under my breath, my head pounding. "I knew this mana-based Kido would be draining, but I didn't expect it to take so much out of me." I stumbled slightly, struggling to stay upright. "I guess this is why Lolo is always harping on mana control," I mumbled, barely able to stand.

As I started to sway, my vision blurred, and I collapsed into the arms of one of the guards, who rushed to support me.

When I regained consciousness, the first thing I felt was a heavy weakness that clung to me like a second skin. My body ached, and there was a constant pull on my right side. As my eyes fluttered open, I realized I was back in my bed, the familiar surroundings of my room calming my panic.

There was no way I could've used up that much mana—wasn't I supposed to have more control? "I must've messed up somewhere in the calculations," I thought bitterly. "That Kido was supposed to be refined, not so draining... but I guess I'm still not accustomed to using so much power."

As I lay there, my senses slowly returning, I heard a soft sound beside me. My eyes shifted to the right, and what I saw brought warmth to my chest.

Rias, my twin sister, had fallen asleep while hugging my arm. Her crimson red hair framed her face, and a faint smile played on her lips. She must've been so worried after I fainted. It was moments like these that reminded me of how different my life was now, how much I had gained in this world compared to my lonely past. I had never known a family in my previous life, and now, I had this love, this warmth.

I gently rubbed Rias's head, and as my fingers brushed through her hair, I whispered to myself, "I'll get stronger. I must. I can't afford to be weak, not with a family like this."

Six Months Later - Gremory Training Area

Time passed quickly, and my progress didn't slow. I had grown stronger, and with that growth, I was eager to test my limits once again. The three guards stood before me, their expressions determined, knowing this would be a test unlike the last one.

"Alright, are you ready?" I called once again, my voice filled with confidence.

"Yes, young master!" they replied, their tone more serious than ever.

I wasted no time. In an instant, I dashed toward the guard on my right. With a simple gesture, I raised my left index finger and declared, "Shō!" A burst of force, akin to a shockwave, blasted from my fingertip, overwhelming the guard's barrier and sending him flying.

Before the other two guards could react, I used the momentum to leap into a defensive stance. They flanked me, one on each side, and prepared their attacks—demonic power crackling in the air. But I was ready.

"El Escudo!" I chanted, and a hexagonal barrier of light green energy enveloped me. The demonic attacks from both guards hit the barrier with immense force, but it held firm, the energy flickering slightly as the blasts were absorbed.

The guards were stunned. I could see in their eyes—the surprise at how effortlessly I had formed such a powerful defense. But I wasn't done.

Next, I lifted my right hand and summoned a burst of yellow energy, forming a crackling sphere of power in my palm. "Bakudō," I muttered.

A glowing yellow triangle formed, and from its three points, smaller triangles of energy shot out, slamming into both guards with pinpoint precision. The force was so overwhelming that they were pinned to the wall, each guard held in place by the energy triangles in a perfect formation.

I stood tall, looking at the immobile guards. "It looks like I won," I said quietly, satisfaction blooming in my chest. "Finally perfected this new Kido variant, mixing it with my demonic power. It's a lot stronger than before."

Meanwhile, in a room within the Gremory castle, two men—Zeoticus Gremory, the Duke of Gremory, and his son, Sirzechs Lucifer, the current Lucifer and former heir to the clan—watched a recording of Maxius's battle.

Zeoticus, his face thoughtful, turned to Sirzechs. "So, what do you think of Maxius's performance?"

Sirzechs smiled, leaning back in his chair. "I believe he will grow into someone incredibly strong, even without the power of destruction. His control over his magic and demonic power is impressive for someone so young. But I can't help but wonder—where did he learn those particular spells? The magic he uses seems so unique."

Zeoticus nodded in agreement. "Yes, I've noticed that as well. He spends so much time in the library, but even so, I didn't think he'd be able to master this kind of magic so quickly."

At that moment, Zeoticus pulled out a set of photos. "Speaking of which, did you see these photos of Maxius and Rias? Aren't they adorable?" He passed them to Sirzechs, eager to share his pride.

Sirzechs chuckled and pulled out his collection of photos. "Adorable? Look at these," he said with a grin, showing off his pictures. "Mine are way cuter!"

The two laughed together, sharing a moment of pride for their children.

One Year Later

A lot had changed in the year since that training session. My magical abilities had grown, and so had my desire to create and innovate. After my spar with the guards, I asked my father for permission to set up a lab in one of the unused rooms of the castle. He agreed, with the condition that I not hurt myself, which, in hindsight, seemed like a good idea.

The technology in this world was far more advanced than anything I had known, and my fascination with it only grew. I spent countless hours in the family library, studying not only magic but also the scientific principles behind Devil technology. Meanwhile, in my lab, I conducted numerous experiments, combining magic and science in ways that had never been done before.

Whenever I created a new spell, I would usually test it on the family guards, much to their dismay. After a while, their complaints reached my father, who told my mother—leading to a rather severe punishment. "Moms are scary," I thought, remembering the look on her face.

I eventually turned to Grayfia, my sister-in-law and the head maid, for sparring matches. Testing my new spells on her became a routine, but let's just say I often found myself embedded in a wall after our bouts. Despite the pain, I knew it was for the best—it was how I honed my abilities.

Another thing that changed was Rias. She had become a constant presence in my life. Whenever she wasn't busy with her own affairs, she would come into my lab and drag me away to play. When her friend Sona couldn't visit, I was the one she turned to for company.

Maxius's Lab

As I stood in front of the incubator, watching the white egg with green spots inside, I couldn't contain my excitement. "The experiment should be a success," I said, practically bouncing on my heels. "I can't wait for it to hatch."

Just as I was about to make another note in my research journal, a familiar voice interrupted my thoughts.

"Hey, Maxius, come play with me!" Rias's voice was cheerful and playful as she entered the lab, her eyes immediately locking on the incubator.

"Rias, not now!" I sighed, but I couldn't help but smile at her antics. She was a constant reminder of how far I had come. She pointed at the egg.

"What's that?" she asked innocently, stepping closer to inspect it.

I grinned proudly. "This is a new species I created. It should hatch in a few days."

"A new species?" she asked, her eyes wide with amazement. "Really?"

"Yes," I said, excitement building in my chest. "I call it a Devi-Beast."

Rias's face lit up. "That sounds amazing! Can I help you take care of it?"

I chuckled. "Of course. I'll need all the help I can get."

And so, my experiments continued, my magical abilities growing, and the bonds with my family strengthening as time went on.