The days following him having caught Aurora in the basement, presumably undergone some kind of forced long-range teleportation, were tedious to say the least. He couldn't tell Rias or her peerage anything, and instead reported directly to Sirzechs. His parents had to undergo memory-erasing techniques to make them forget about his use of magics.

Issei had recovered well. According to Elsha and Ajuka, his brain had fully healed and was operating even better than it was before. They each theorised that it was because of his repeated contact with higher dimensional constructs. His body had indirectly prepared him for the next encounter better; a vivid demonstration of evolution in a microscopically smaller timeframe. Regardless, he was still suffering minor hallucinations with no known cause.

Sirzechs and Ajuka were more than extremely concerned about what he had done and seen. They practically demanded to go into his basement to see for themselves, accompanied by an army, but Issei's unrelenting refusals had convinced them.

He had potentially the strongest mental barrier of any living creature, thanks to his past traumas and experiences combined with his shared mind between Belzard, Elsha and Ddraig. They took minor portions of each attack on his mind and greatly reduced the impact, and yet he was helpless in front of the concepts. If the devils saw what he had seen, it's not unlikely that they would succumb to madness. Elsha had also speculated that he was so vulnerable to the following psychological assault due to his previous contact with Infinity, alongside the lack of Ddraig and the others to take a load from him.

He was already lucky enough to come out of it as unscathed as he was. If Ophis didn't want him to live for its entertainment, it could have ended his existence without a thought. The Unknown Concept, as Ajuka had named it, seemingly didn't want to kill him outright either. Or it couldn't, for another unknown reason.

Issei had to teach the beholders of the Thrones of Hell what concepts were. Previously, he had learned from Odin and the Ouroboros. Odin was determined to take the knowledge to his deathbed, and only told him the nature of the strongest powers while in a puddle of his ichor. Ophis had told him in passing during some of the first attacks of the mirages. It was a joint assault with the Khaos Brigade, its very own terrorist force, to weaken the defences of the Grigori and the Underworld while the machines attacked Heaven simultaneously.

Since then, Issei had used and abused any vow he could make with the World. The Concept of Life operated on an exchange basis; it would allow any being with life to make a pact with itself in exchange for power and a sacrifice made. The sacrifice could be something as simple as a statement such as "I will only use my left arm for this attack", and the resulting attack would be significantly empowered.

Yet another reason why he was the last one standing.

He did not tell them about the details of each concept, only their existence and nature. It almost immediately reminded Ajuka of the Trihexa and, upon his resulting question, was met with an "I don't know" from Issei. Concepts weren't quite alive and, as the original God had proclaimed once, the Beast of the Apocalypse was an extant form of life. A twisting malformed life form, with a poorly-defined nature, but a life nonetheless.

Another thought came to the Super-Class devil, that of the necronomicon. It was a book bound in human skin containing countless knowledge of magics and stories of the Old Ones. Ancient Outer Gods that, while can be summoned, spread madness and manic hysteria amongst their ritualistic followers. The necronomicon was passed to the Thrones of Hell from the Original Lucifer and kept safe within their reach. Issei didn't make the connection between it and the concepts.

Ajuka, however, was possibly the most intelligent being alive. He had more knowledge than most gods on their own domains, and could reduce any spell into simplified mathematical equations before rewriting it and nullifying it. He was proposing that the book had knowledge on numerous forgotten gods, and the identity of the Unknown Concept may lie within.

The thought that he may be fighting an Outer God worried Issei immensely. They were unrecorded existences close in power to the Heavenly Dragons and the Biblical God while only being partially called upon. If one had fully descended, he would need a lot more preparation than expected.

For now, Ajuka promised to look into it for him. Even the Lucifer Blue was hesitant to read the manuscript, it had countless indirect traps. So much as reading it aloud was dooming you to a fate worse than death. Saying the name of an Ancient One was prompting it to look at you, and bearing attention from the eldritch always lead the beholder down a path of perpetual torment.

It would take a long while before any results would be concluded from searching, but it would be done.

In the meantime, Issei had further reinforced every defence mechanism within his home. He had borrowed Ajuka as a magical battery in exchange for allowing him entry and letting him see the barriers in place.

To the Satan, it was like being a child in a warehouse of various candies. Magics he had never seen before were weaved seamlessly into existing formulae and perfectly complimented each other. It was incredible. A barrier of holy affinity that even he couldn't read, numerous anti-teleportation fields, illusionary barriers and walls that even deceived him along with so much more.

For the first time, he had seen something entirely unique. Issei, who was in the form of a blonde woman bearing blue eyes, had shown him a responsive barrier. Initially, it didn't exist until a force was demonstrated. Then, it reflected the force hundredfold. It was different to reflective barriers since it was completely undetectable in every possible way beforehand.

It was like this woman, who had called herself Elsha, had completely erased the mathematics behind the magical construct. It clearly existed since the spell worked and had a degree of sentience, and Ajuka could've sworn he had just fallen in love.

Her creating a barrier that rotated in numerous unpredictable directions only solidified this thought. It was akin to a meat grinder, using another barrier that didn't exist to manifest behind the attacker and, quite literally, grind them into dust. He had tried seeing the mathematics behind the rotation and was entirely stumped.

While magical formulae were notoriously long and detailed, he could decipher them perfectly. This one in particular had a longer formula than the space it occupied, which defied more than one key law. It used imaginary fifth-dimensional matrices containing non-homogenous partial differential equation systems in each element to describe the motion between a flowing time, three dimensions of motion and an included dimension to completely alter the variables and their own derivative systems within further fourth-dimensional matrices using even more derivatives and matrices in layers he couldn't so much as count. According to Elsha, there were a constantly changing number of equations, elements and variables, each of which had a constantly changing order that reached a maximum of fifty one and a minimum of twelve, which was mathematically-

"Are you done?"

He was tired of sitting there watching Ajuka stare into space mumbling to himself. He had brought a notebook and completely filled it, written on the space between them, then began writing on his arms and legs. He had started writing on Issei too, which was a bit too much.

"No. Issei Hyoudou, how much money do you need to sell me Elsha? If you need my mind, body and soul, I will gladly give it to you."

"Fucking excuse me? She's attached to my soul too, good luck with that."

You would have thought that he had just forced Ajuka to kill his entire bloodline alongside every friend he's ever made, then promise to quit studying magic. The look on his face before was countless shades darker than obsessed, and it almost looked like he was going to kill himself upon hearing the knowledge.

Ignoring him and leaving him to his wallowing pity, Issei admired the handiwork. Elsha was as incredible as always, and had taken the time to study further for him. She had even made it so whoever was marked previously could still pass through every additional procedure. He had a few ideas in mind for magics that he may need, so he would rely on her even further.

Seeing the effort and work she had put in for him warmed his heart. She had nothing to gain in particular, and yet her kindness was her greatest asset. Azazel may argue about other assets, but Issei would gladly throw him into the ninth circle of Hell for the comment.

He was also just glad not to have any school, since it was the summer break, but it also meant that the gears of war would be turning too soon. Leading the dejected Ajuka downstairs to his basement floors, he threw him into the magical workshop and left him there to make his notes before going down to the swimming pool. There was a certain kind of training he had to conduct.

Rias, Akeno and Koneko were looking for Issei. He had brought Satan Beelzebub over and was increasing the defences, then they had both just disappeared. Through the false wall and down the spiraled staircase, they were surprised to not find him in the training room. Taking the lift down to the first floor and upon seeing Ajuka sitting atop a tower of papers, still writing and adding to it, they had turned around and left.

Upon reaching the second floor, they could tell he was in here. The usually hospital-white lights were dimmed into a deep red. A quick search revealed nothing, but upon checking the sauna they had found him completely nude and horrifically burned. He was deep in meditation and upon watching him, they found his burn injuries would be healed with magic, inflicted again and healed again. Once the burns weren't significant enough, he would heat up the room even further and repeat the process.

It looked like self harm to them. The air was too hot for them to reach in and pull him out, but simply calling out to him wouldn't stir him. They could fire magic at him, but if he was startled enough by it, they would likely be eviscerated before he even acknowledged a threat had existed.

Koneko didn't care, or didn't recognise this. She had walked in with her head down, grabbed him by his hair and threw him directly into the swimming pool. As a Rook, she would naturally suffer the least damage from such a thing, but it was hot enough to severely burn an Ultimate-Class being. She was at the strength of a Middle-Class at the highest.

Issei had gone into the magically enhanced sauna and started his training. It seemed utterly pointless at first glance, but it was the perfect method. He would burn his skin horrifically, heal it to the point where a scar wouldn't form and repeat. He had spent, according to Ddraig, seven hours constantly torturing himself before his sensations changed completely. A small hand had grabbed his hair and gravity flipped upon itself.

Swimming out of the pool and shaking his head, he saw an unconscious and seizing Koneko entirely covered in charred black skin being treated on the floor by a panicked Rias and Akeno. He was there within the instant, and they parted for him with tears stinging the redhead's worried cerulean eyes. The lights dyed themselves white once more.

She had went in to get him.

Shirone's clothes had been reduced to ash within the instant, her skin had not only charred and was sliding off her skeleton but had burst in certain places. Her blood had literally boiled her internally and her brain was turning into mush. The burns were much more than her entire body, they had burned her organs too.

Boosted Gear manifested, she was encompassed in healing spells stacked upon each other. They aimed to fix everything except her brain immediately. Elsha helped him to create a High-Class freezing spell, and he submerged her in the rapidly cooling water nearby. The healing spells did not stop.

It was like seeing a miracle take place for the other two devils. They had seen his healing countless times before, and it was never this efficient. Where it was once a Low-Class spell, it now had the proficiency of a Middle-Class while maintaining the number of circles. Koneko was literally being created a new shell of skin and having it grafted on, all the while her organs and veins were expelling the destroyed cells and creating new ones.

It had taken only ten seconds before her body was entirely perfect and unmarred once more. Issei didn't so much as spare a glance at her naked form, instead turning his attention to her head.

While her skin and eyes had been entirely replaced, her hair could wait. He needed easier access to the brain, which was a far more delicate operation.

[BALANCE BREAKER]

The crimson armour completely wrapped around him after an aura of green light blinded his witnesses. He removed Koneko from the water and dismissed the portion that covered his bare front half. Holding her seizing form upright, he embraced her. A faceless undetailed spirit with a single pair of glowing vermillion eyes manifested behind him and walked around his side until it stood behind her. It held her closer to him, embracing them both with its elongated forest-green arms.

Issei's arms wrapped around her, refusing to let go. Ddraig's commands were accelerated within his mind, clearly desperate too.

[OVERTUNE]

The doubling had ceased. With stolen power racing through his worn body, three larger circles were formed and he began stitching each neuron back together. He reformed mental connections, created new cells from bone marrow and did his best to maintain her memories. Issei knew it was working when her violent twitches had stopped, but he refused to stop even after he thought he was done. When all the connections were reformed, he continued making more and more cells. If he had missed a spot, the excess would hopefully compensate.

Issei did not relent until the power he was granted dissipated. His armour fully covered his exhausted form but the spirit had faded into the wind. Koneko lay atop his chest, with her heart beating and her lungs performing.

The timer for him was ten minutes, double the length every other user had, but it had drawbacks. It was an oath made with the World, after all. In exchange, he could no longer use the basic command [EXPLOSION] to release his power. Instead, he had to designate the power into a specific aspect of himself.

[OVERCLOCK] placed an emphasis on his mental capabilities, [OVERWHELM] did so to purely his physical strength, [OVERTUNE] perfectly controlled his magical capabilities and [OVERDRIVE] was a balance between his reflexes and all of his physical attributes. It benefited him even more, since he preferred being perfect in one aspect as opposed to mediocre in everything. At least, he did back when the pact was made.

Now, he strived for perfection in every aspect to maximise the efficiency of the commands. The stronger his base, the stronger his empowering.

Koneko was perfectly healthy, she just needed to rest for now. It seemed a better time than any to stop his training, but he couldn't stop the nagging feeling within his heart that it was his fault. Rias and Akeno seemed a lot less concerned once he had stood and picked her up. Upon seeing them pan their gaze down from his eyes, he moved his carried wife forward slightly and looked down.

Issei was still naked, having dismissed his armour when standing up. He made sure to hold uncomfortably long and forced eye contact with each of them before smiling something devilish and taking her to the lift. They didn't seem too keen to get in with him for some reason, maybe they needed a minute to breathe.

Koneko was placed on his bed and he continued monitoring her with magic. He had finally put some clothes on after Kiba and his parents had seen him striding around bare holding her. It was only awkward on their end, he didn't care too much. Issei was resting downstairs when Ajuka had emerged out of the false wall. He looked exhausted, frustrated and tired, nothing the Head of Magical Engineering couldn't handle of course. He had come to ask for permission to install a linked teleportation sigil within his workshop.

Conditionally, of course. A deal with the devil was only made when you had something to gain. Issei wanted to borrow his magical reserves another time to further bolster the defences. Ajuka, hearing this, was a little bit too excited to get it done.

Once he had added it and left, his parents had prepared dinner for them all. It was an awkward affair for the present devils for some reason, not that he minded too much. They all looked like they had fevers, minus Kiba.

When the time to sleep had come, he asked Rias to sleep with Akeno for the first time in a while. Usually, the sleeping arrangements followed Kiba sleeping in a spare room, Akeno in the same situation and Rias sleeping in his bed alongside Koneko. While he absolutely loved the arrangements, the white cat had to be monitored. She understood, even if she didn't like the sound of it too much.

According to herself in the past, she struggled to sleep if she wasn't nude and if she didn't have something to crush in her arms. Issei was happy to take that role alongside his wife, but his other wife's health came before comfort.

The monitoring was up-kept by Elsha while he slept, training further within his mind. Today had trained his healing magic, pain tolerance and heat resistance. Now he would further his creative use of magic. Ddraig would place him in certain near-impossible situations and, forbidding the use of any magic besides one, would tell him to escape alive. Training with the Heavenly Dragon had its advantages, especially since he couldn't die within his soul.

Upon waking up to Koneko still sleeping, he continued about the new day. He made sure she was hydrated and satiated, but when she was still asleep the next morning he started to worry.

She wasn't waking up.