(I will say DxD, Rosario Vampire, and Testmant Of a Sister New Devil all start around the same time with some wiggle room. Around 2006-2008. I rewriting this to remove the Boosted Gear. I have done it too many times and I got bored fast. Now, let's work with good old Apophis from DxD.)
In Tokyo Japan, in 1996 a small boy had just left his home as it was the weekend at the moment. He wasn't Japanese as he was white and was just in Japan with his parents for his Father's job.
He stood out as his hair was naturally dark blue with a few black outlines in it. Most people were born with common black or brown hair, but some just had unique hair colors like him. However, he did have one issue plaguing him at the moment.
He didn't like it here as Japan was weird when compared to Texas and he wanted to go back to his friends. However, his Mother and Father liked it here so he was stuck until they had to move back.
He sighed as he closed the door behind him as he barely knew Japanese. He could hardly read the stupid language.
"I hate it here."
His name was Jack Owen Jr, currently eight years old, but his friends always just called him Jack'O. Too bad he didn't have any of those anymore as it was hard enough to make friends as a foreigner with how racist Asians were.
They even had a word for foreigner which was Gaijing. His Father, Jack Owen Sir told him to punch any kid who dared call him that. He had taken it to heart and now he was distrustful of Japanese people.
He put his hands in his Jacket as it was late December and it was snowing. He walked away from his house as despite his dislike of Japan, it was at least fun to walk around in the snow.
As he began his walk he thought back to his time in Dallas. While some parts of Dallas were dangerous all cities had their rough parts. He was lucky his Father got paid well so he lived in the upperclass neighborhoods.
He did miss the food from Texas especially a Gas Station that sold really good Mexican Food. He smiled to himself as he thought back to the good old days of Mexican Tacos or Tortas.
Just as he was thinking, a snowball hit him in the face. He snapped out of his thoughts as he turned to the direction it came from.
"HEY!"
He heard a familiar giggle before he saw one of his only friends he had made since coming to Japan. Aya Akagi with long red hair which contrasts with his blue hair.
"You had that stupid look on your face again."
He was lucky that she knew English, and was nice enough to help him study up on his Japanese.
"And you still look like your hair is Kool-Aid."
"And you look like a blueberry."
He bent down and packed some snow together before throwing it and hitting her with it. She chuckled as she ran over to his side.
"Nice one, Jack'O"
He smiled to himself as he kept walking now with his friend.
"Thanks. What's up?"
She shrugged.
"Nothing, since it is Winter Break I thought I should stop by to see you. Knowing you, you would have been all alone."
He hummed.
"Thanks, I guess, it is hard enough to make friends in a new country much less learn a new language."
She nodded.
"Which is why you should be happy I am helping you out. You can bow."
She closed her eyes and stuck her nonexistent chest out. Instead, he bent down, grabbed some snow, and stuck it into her jacket.
"HEY!"
While she thrashed around getting the snow out he cackled as he ran off.
"THAT IS WHAT YOU GET!"
"I WILL GET YOU FOR THIS JACK!"
She ran after him, but with the snow, it was not a good idea to be running. As he was running, he slipped on the snow and when he fell he hit his head pretty hard against one of the stop signs.
The hit was so hard that Aya froze in shock.
"JACK!"
She ran over to his side to see if he was ok. However, he was bleeding from his head so she freaked out.
"HELP! SOMEONE HELP! CALL AN AMBULANCE!"
She started screaming quite frantically and it caused some people who were walking by to try to offer their help. As for Jack, when he hit his head he almost died with how hard he hit his head.
The instant that Jack nearly died, one of his multiversal alternatives died to enemies stronger than himself. The nature of his death caused him to be sent hurling through dimensions until by the control of a higher power his brutalized soul merged with his alternative self Jack.
As the two Jacks began to fuse into one being, the second brought something along with him. A Longinus, but not one of the original 13, a new one created by Azazel, and Ajuka. The Abyssal Eclipse Gear contains the soul of Apophis The Primal Eclipse Dragon.
When the Evil Dragon the mortal enemy of Ra the Sun God awoke and felt what happened he was intrigued. He could sense that his new host was in a near-death state. As such, he began to use the limited amount of control he had to make sure this fusion went well.
It was enough to keep Jack alive until the merger was complete and he was taken to the hospital. At the hospital, ack received proper medical attention which saved his life.
Apophis seeing that most of the danger was gone, sighed to himself before relaxing. He could speak to his new host once he woke up as it was pure luck two Jacks had merged. Regardless, while Jack was out of it, he began to experience the memories of this other Jack.
Him being born, how he was born with a Twice Crital. How he trained like his life depended on it for years until Issei defeated and killed Apophis. Then, the soul of the Evil Dragon was sealed away and taken to Grigori.
Their, Azazel and Ajuka pulled their genius together to create a new Sacred Gear. Something that could rival the Longius, the Abyssal Eclipse Gear. The best host was Jack as he had a Dragon-based Sacred Gear.
Neither Issei nor Vali could do it so he was chosen. After his Twice Critical and Abyssal Eclipse Gear merged, he was changed. He rose in power quickly reaching Issei and Vali in power, but it was for nothing as he died in the battle against the ExE gods.
All those memories and reliving them was a lot to take in. When Jack opened his eyes, his eyes seemed to shimmer a dark black, but it was gone when he blinked. He stayed still as his head was wrapped in bandages, and he was attached to many medical devices.
Jack was really confused as he began to call back different memories he shouldn't have known. Not only did he gain the strength of his previous self's soul, he also gained his Mana.
Meaning a normal eight-year-old boy now was overflowing with high levels of Mana. At the oldest, he was only 19 when his other self died, but he had achieved a lot in that time.
He wanted to sit up, but he felt a weight on his chest. When he did so, he saw his Mother resting her head on his chest. Her long blue hair was spread out on the bed and he could see she was crying.
"Mom?"
When she heard his voice she shot awake.
"BABY!"
She seemed exhausted, but happy that he was awake.
She held his face before kissing him on the forehead.
"Don't move, I will get your Father and the Doctor."
"Ok."
She got up and left the medical room to inform her husband and the doctor of his waking up. When she was gone, Apophis took the chance to make himself known.
[Honestly, what a foolish boy you are. You almost killed yourself by slipping on snow. It is almost funny.]
Jack looked around the room but he didn't recognize this voice.
[I am not in the room boy, I am in your right hand.]
Jack raised his right hand and he saw what seemed like a dark purple glowing mark on the back of his hand.
"Who are you?"
[You should know already, that I am Apophis. You did just get a big upload of memories yes]
Jack thought back to the memories of his other self before his eyes shot wide open.
"Apophis, the Evil Dragon."
Apophis chuckled.
[Yes, that is me, as you can see, I was defeated by that boy, Issei Hyoudou. For all his perversion, he was an honorable warrior. I would have fought him again, if not for that White Cat.]
From what he saw, the cat, Shirone or Koneko used her Senjutsu to seal Aphophis into one of the Gems of the Boosted Gear. Then his soul under the work of Azazel, Ajuka, and the many geniuses of Grigori worked on the Twice Critical of Jack.
They used the base Twice Critical containing a portion of Ddraig's power made of his flesh, the Gem that came from the chest of the Diabolos Dragon God Armor, and Apophis' Soul to fully evolve the Twice Critical of the other Jack.
This resulted in not just the Twice Critical evolving into a Longinus-level Sacred Gear, it kept its base abillities. This means that not only could he use The Boost to a limited degree, but he could also call on Apophis' Abillities.
If not for the limited amount of time the other Jack had, he would have become a monster. Just his time was cut short by foes stronger than himself.
"This is a lot to take in."
[I can see that. I too was surprised to know that after my previous host's death, he would fuse with a little boy. No matter. I might as well make the most of it. My previous host was quite strong for only having a Twice Crical and only having a single year to train with his new Sacred Gear.
If you don't measure up to him, you will shame his memory.]
Jack wanted to keep talking, but at that moment his Mother returned with his Father and the Doctor. His Father when he saw the state he was in his stern face seemed to soften. He walked over before he placed a hand on his shoulder.
"How are you feeling?"
"Good, I just have a headache."
The Doctor walked over and he began to check him over.
"It is a good thing he got medical attention when he did, he could have died. He has some light skull fractures and a concussion so he will need supervision until he fully recovers. However, he should be out of danger."
Jack Sir nodded upon hearing that.
"That you, Doctor."
The Doctor turned to Jack before speaking to him.
"In the future be more careful child. You almost lost your life."
Jack nodded.
"I will."
"I hope for your sake and your parents you keep to it."
The Doctor then left the room leaving Jack with his parents. His Mother sat by his bed before holding his hand.
"You had us worried half to death. It is a good thing Aya was with you when you hit your head. She told us you slipped on the snow."
He nodded.
"I did, I played a prank on her and started running. I slipped and banged my head on a pole. How is she?"
His Father sighed.
"Scared you were going to die. She was crying and apologizing to us saying it was her fault. We know it was an accident, but be more careful son. We almost lost you and you all we have."
Jack felt bad not just about almost dying, but worrying his parents.
"Sorry."
His Father shook his head.
"Don't be, you have never seen snow before so it was obvious for you to be excited."
Jack nodded as he would be more careful from now on. He stayed in the hospital for the rest of the night before being discharged in the morning. His parents were extra watchful of him to make sure he was ok.
While his Father was at work, his Mother paid more attention to him. He was fine with that, but the night after returning home it was time to get his situation sorted. He sat up in bed the next day after reawakening his other self's memories and the Abyssal Eclipse Gear.
He placed his right hand on his lap before he closed his eyes and focused on his other self. How to call on his Sacred Gear which he had done hundreds of times since he was a child.
He focused on his right arm and pictured his Sacred gear and just as he wanted a large Abyssal Black gauntlet appeared with two large gems. The main one was like liquid darkness was contained inside the other was a deep emerald.
"It was real."
Apophis felt amused at this.
[Of course, it was real. I am here, aren't I? Those memories are here too. But this is a good thing, with the memories of my old partner you should know what you right?]
"I think so, this will make me super strong right?"
[It will. If you aren't a failure, you will certainly become powerful. The best thing is, I am stronger than I used to be. Ask me why?]
"Why?"
[Glad you ask. As you know, this gauntlet used to be a Twice Critical made of the flesh of Ddraig The Welsh Dragon. Jack used it all his life and even evolved it into a Sub-Species, but for all that, a Twice Critical is still a Twice Critical.
That all changed when my soul was trapped in the main gem of the super-powerful mode Issei Hyoudou used. The power of not just Ddraig, but Ophis and Great Red in small amounts was inside.
That gem which now contains my soul was used to enhance his Twice Critical into what you now see before you. The Abyssal Eclipse Gear is formed of the combined genius of Azazel and Ajuka. Why this matters is, I gained the Boost of Ddraig.]
"Awesome."
[Exactly. Sadly, it isn't perfect. Rather than Boosting every Ten Seconds, you will only be able to do it every 20 and it will take a lot of energy from me since it isn't my natural ability. I think at the moment the most you can do is four maybe five.
That may change in the future though. Why not give it a try?]
Jack focused on his Right Hand on the dark gauntlet before doing what Apophis said.
[Boost.]
The voice of Apophis emerged from the Black and Red gauntlet causing the black gem to glow a deep purple. When he did that, he felt his physical abillities all double. His strength, speed, durability, his thoughts, and that wasn't all. He could feel something stir from within which Apophis explained.
[The Boosted doesn't just double your strength, speed, durability, or magic. It can even double your recovery speed and even the speed of your thoughts. That is why fighting Ddraig is so damn hard, as that bastard can do it endlessly. Now, his power is mine, be thankful.]
That meant that enough boosts and Jack could temporarily become super intelligent. It would make studying easy.
[Not just that, by Boosting enough you could even create your some deadly magic. I can teach you some. If you want to be the new Eclipse Dragon Emperor. The first in this world. Do you want that?]
Jack nodded.
"I do."
[These perks come with a cost, you will have to face the dangers of this world. You can see from your memories yes? It won't be pretty and it will be challenging. If you give up though, I may self-destruct to take you with me.]
Jack got out of his bed before he held the Primal Eclipse Gear. 20 seconds had passed.
[Boost.]
With that action, his attributes quadrupled causing his body to be covered in a deep black and crimson glow.
'I want to be stronger. Teach me how to be strong Apophis?'
[You may yet have merit. I shall as I have nothing else to do, partner.]
[Boost.]
With his abillities increased by a factor of Eight, he began to actively recover from his concussion. His thoughts were much faster, he felt invincible and this was just three Boosts. He imagined what 100 would do and he could not wait.
'I will be the strongest.'
Apophis was already starting to wonder what this new Jack the result of two souls fusing would achieve. Just maybe this Jack would follow the path of the Evil Dragon.
(Someone commented that I already used the Boosted Gear and I did do that. Time to add Apophis and his Darkness powers into this new world. New Longinus with the stolen power of Ddraig.)