What was his system?
Isaac had to be a fool if he said that he hadn't asked himself that question at least once before.
He knew it was something not native to this world, nor was it something that should persist in it. And yet, taking him in as the host, it had bonded with him to continue its existence.
Still, that was just speculation.
He doesn't even know its core essence, where it came from, and why it specifically picked him, of all people.
All he knew was that it was unique to him, thus why he called it a Uniqueness.
'To be honest, it was suspicious how I've never once received any kind of penalty from the system...' He thought to himself as he recalled the nature of it. 'From the missions to even the items I bought, they all don't possess any inherent penalties, except...'
Except the rewards themselves.
Some have downsides to them to be paid in exchange for utility, while some are just outright a borrowed gain for an exchange of penalties.
The more Isaac thought more about it, the more it had put him on his edge.
'The system had not been giving me any penalties... Because the rewards themselves are the penalties in disguise.'
Why else would he be given missions that were intended to push him from the very thing he had sought after, which was peace? It was called the Serenity System, after all, not the Adventure Quest System.
He grimaced at this revelation. 'It truly is a masquerade of the guilty...'
With this revelation, he couldn't help but feel reluctant about the new quests he had just received.
Noticing Isaac fell into silence with a grimaced expression, Nahida caught on to this and spoke.
"You're quiet. Is it perhaps because you know something?"
"Sort of?" Isaac shifted his gaze back at her and replied with a wry tone. "I mean, I'm still unsure, but... There's a non zero chance that I might know where that influence is coming from."
Nahida seemed to contemplate over something for a moment, her expression falling into that of a languid one. Then, as if she had connected the dots in her mind, she spoke.
"I see... Then, if you already know the source of this speculated Abyssal influence, we might be able to help you sooner."
"Really? How so?" He asked.
Nahida heaved a brittle sigh. "Regrettably, I alone may not be able to do it. Anything related to the Abyss is bound to be a malignant force that only seeks to corrupt and infect anything around it while simultaneously being highly resilient to any conventional means."
"Unless we have someone or something that is capable of drawing out that influence inside of you and using Light Realm energy to cleanse it, I don't think we would be able to heal you anytime soon..." She added with a forlorn tone of voice.
'Drawing out...' Remembering a certain someone capable of doing such a thing, Isaac immediately asked her a question. "Then, what would happen if we just draw out that influence?"
"No, that's not something I would advise doing." She frankly denied with a shake of her head. "Drawing out such power would only result in complications. To put it into an analogy, it's akin to plucking out the trunk of a tree while the stump stays behind."
"Dealing with the Abyss needs it to be dealt thoroughly. If not, then it would only grow back."
"Alright, I get it." Isaac sighed. "So we can't just brute force it out of me, it needs to be removed entirely and purified, then."
Nahida nodded. "Yes, and so far, only the Light Realm and its derivatives, which are the Seven Elements, are capable of combating the influence of the Abyss. The Heavenly Principles designed it so that the world can have a chance at fighting the Abyss in their absence."
If the Abyss, also known as the Void Realm, was an all-consuming tide that ravaged the sea of stars outside the world of Teyvat, then the Light Realm was the opposite. The two were the complete polar opposite, and such, naturally counteracted each other.
It was only after the Heavenly Principles designed the Seven Elements did the Light Realm gain another boon against the Abyss.
'I was originally thinking of just asking the Traveler to pluck out the Abyssal influence that the system had given me, but since it now needs someone to cleanse it...'
There was only one individual Isaac was thinking of that fits the bill.
The only problem was finding that person and convincing them to help him in his dilemma.
His gaze shifted slightly and met the system interface to look at a certain quest he had just been given.
[Mission: The Knight of Witches]
'...if I want to meet that person, then there's no better way to do it here.'
The name of the mission alone pointed to a certain organization.
He was, of course, thinking of seeking the help of a certain angel that was closely tied to the Heavenly Principles, and by extension, methods of cleansing the Abyss.
And while he was at it, he would also finish the assigned mission to him there.
'I was already planning to find her to ask if she knows anything about my condition, but to think I'd still end up seeking her for another matter...'
What a coincidence it was, truly.
Having thought it through, Isaac turned his attention back to Nahida. "Looks like we don't have to wait for too long. If luck had it, I might be able to secure that person who can help the process."
"Hm, well, if you are sure about it, then I can only wait for the good news." She said as she made her finishing moves on the chessboard.
He controlled his own pieces on the board, but even though the game was far from over, he knew that he was gonna lose soon. Knowing this, he merely smiled. "Alright, I'll be sure to meet your expectations."
Nahida thought for a moment. Then, with a curious look, she asked, "Now I am curious. Who or what is it that can help you with this? There are very few things in Teyvat that I know of that can safely tackle the Abyss, so I can't help but want to know."
"Haha, you'll have to wait for that." Isaac laughed. "It's no fun knowing everything, you know? I'll introduce you to that person once I convince her."
"Hehe, teasing the God of Wisdom like this, aren't you too much, Isaac?" Nahida chuckled.
Isaac scoffed. "Ah, if only the God of Wisdom could go easy on me in chess. But alas, I have no choice but to be harsh to her."
"Meanie..."
"Ehe."
Their game continued, and sure enough, Isaac lost in no less than a few more moves by Nahida. He had already anticipated that outcome, so he wasn't quite upset about it, but it did make him feel a bit indignant that he lost to her when he knew she wasn't even serious.
After their game, as Isaac was packing up the pieces, Nahida looked at him in silence for a bit before speaking.
"By the way... Are you interested in visiting Sumeru by any chance, Isaac?"
"...?" Turning to her, Isaac raised a brow. "I don't have any plans anytime soon, well, I am planning to, but not right now. Why do you ask?"
She lingered for a moment then shook her head. "It was nothing too serious. I was just thinking that introducing you to Sumeru's culture would be a good way to repay you for all you've done to me..."
"Haha, really now?" Hearing her reasons, he couldn't help but chuckle. "You don't have to repay me, you know? That was my decision to do, so there's really no need to do anything for me."
"So long as you're having fun, comfortable, and most of all at peace, that's all that matters to me."
"Alright." Nahida accepted it rather quickly. Then, she hummed a soft tune that Isaac often hummed. "Then, I'm offering it to you not as repayment, but an invitation."
After saying that, Nahida clasped both of her hands together. Then, drawing from the elemental energy around her, an item materialized out of the elemental particles gathered.
After coalescing into a solid material, Nahida looked at it for a moment before nodding in approval. She then handed it to Isaac
"Should you ever want to visit Sumeru, just wear this and I'll make sure that you'd have the smoothest stay."
Isaac received the item that resembled an Akasha Terminal and didn't know how to respond for a moment.
But after seeing how insistent Nahida was about it, he found it to be quite rude if had declined further.
And as such...
"Alright, I'll make sure to treasure this when I come over to Sumeru." He said as she grasped it. "Thank you, Nahida."
"Hehe, it's nothing." Nahida replied with a soft smile. "So long as you stay out of danger, then all is fine with me."
"...?" Isaac didn't know why she said that, but he still felt thankful. "Yeah, I'll make sure not to get into any trouble. Especially now that you're counting on me."
With that, he patted her head once more. He had done it so much in the past that it became an unconscious act by now.
And seeing how Nahida didn't resist it and rather looked quite more happy about it, Isaac just continued to do it from time to time.
It had to be therapeutic at this point.