Whether "underground passage" was an apt description or not, Rei and Grimm pressed forward through it.
Both Rei and Grimm possessed Night Vision. Even so, Night Vision was useless in places where no light existed at all. Rei aside, Grimm was an Undead, and as such, could see perfectly well even without a light source.
Noticing Rei struggling to secure his field of vision, Grimm waved his hand lightly. In an instant, a faint Light Sphere materialized in the air.
The sphere wasn't particularly bright—by design. Grimm had kept it dim enough to avoid blinding Rei, reasoning that for someone with Night Vision, this much light would be plenty.
"Sorry about that."
Rei understood that Grimm had created the Light Sphere out of consideration for him, and that was exactly why he voiced his gratitude.
"I do not mind. In this matter, it is only natural that I act alongside you, Rei."
Even as he said that, there was no doubt it was truly out of consideration for Rei. Even if Rei hadn't asked him to investigate, Grimm would certainly have created light for his sake.
Using Grimm's Light Sphere as a guide, the two descended deeper underground. Several minutes had passed since they began walking, yet they still hadn't reached the bottom.
"So exactly how long is this passage?"
"Hmm, that much even I do not know."
"...You're the one who made it, aren't you?"
And yet he didn't know? As Rei pointed this out, Grimm shook his head.
"There is still much about this ability that remains unclear to me. It may also be that I am not yet accustomed to using it. ...Still, I never imagined I would end up doing something like this together with you, Rei."
"Is that so? ...Now that you mention it, I suppose so."
Rei had initially questioned Grimm's words, but given that Grimm was an Undead, he couldn't simply go out and about so easily. If it were just him alone, that would be one thing—but if someone saw him walking together with Rei, it would undoubtedly cause serious trouble.
"In that sense, this was the ideal place. ...Though whether that's something worth celebrating is another matter entirely. Ah, it's not like I dislike being with you or anything, Grimm."
"Umu."
Grimm gave a short nod. Though his expression—more precisely, his voice—betrayed nothing, inwardly he was relieved. Now that he had come to think of Rei as a grandson, being disliked by him would have been devastating.
That said, he had no intention of spoiling the boy. Even so, the fact that Grimm was using magic to investigate the cause of interworld teleportation would, to any outside observer, certainly seem like he was coddling Rei. For Grimm, though, it wasn't about spoiling him—he simply wanted to see an existence that had piqued his interest with his own eyes.
"Grimm, when you were alive, did you do adventuring-like activities too?"
"Indeed. I did similar things. Of course, compared to Lord Zephyle and the others, there is no comparison."
Grimm possessed more than enough ability to be called a genius himself. His misfortune was sharing a generation with Zephyle and the Zephyle Clan—those who should be called geniuses even among geniuses.
Though they were all called geniuses, among those geniuses existed what one might call a higher tier. If hearing one thing and understanding ten made one a brilliant talent, and understanding a hundred made one a genius, then Zephyle and the members of the Zephyle Clan were those who, upon hearing one thing, could comprehend a thousand, ten thousand, or in some cases even more.
Even if everyone above a certain threshold were classified as a genius, the disparity in ability within that group would still be vast. In that sense, it was undoubtedly a great misfortune for Grimm and others who bore the title of genius yet could not match the Zephyle Clan.
"Huh. So Zephyle really was that amazing."
"Naturally. In the first place, none but such geniuses could have created a new sorcery like Beast Magic."
Even as he said this, Grimm sounded somehow proud—likely because Zephyle and the members of the Zephyle Clan remained objects of his admiration. Rei couldn't help but suspect there was some nostalgic embellishment at work, though.
"Then do you think Zephyle could have done something like teleporting to another world?"
While proceeding through the passage, relying only on the small light Grimm had created, Rei posed the question.
From Rei's perspective, the fact that Zephyle had contacted the soul of his past self after he died in an accident in another world—Japan—was an incredible feat. But he suspected that directly teleporting to another world had been beyond even Zephyle's reach.
If Zephyle could teleport directly to another world, there surely would have been other methods besides searching for Rei's soul. ...In fact, while he couldn't speak for other countries, if limited to the region called Japan, Rei was certain that if asked whether they wanted to go to another world, those who would nod would be by no means few. Especially those who enjoyed subculture—manga, anime, novels, and games.
Given that Zephyle had never done such a thing, he might have been able to exist in the Gap Between Worlds—the space between dimensions—but could not teleport directly to another world.
When Rei pointed this out, Grimm, who had been gliding forward just above the ground, came to an abrupt halt.
"I... am attempting something that even Lord Zephyle could not...?"
"Hm? Ah, probably. Though the idea that Zephyle and the Zephyle Clan couldn't travel to another world is really just my guess. But given that Takumu was among them, I think it's fairly certain."
Takumu Suzunose. A member of the Zephyle Clan, and someone from the same homeland as Rei. Judging from the fact that he had been wearing a school uniform, there might have been some difference in era, but he was believed to be someone from the same Japan as Rei.
If it were truly possible to teleport to another world, it wouldn't have been strange for Takumu to want to return home. But since he ultimately never did, it meant teleporting to another world was likely impossible.
"Hmm, I see. That I would attempt what even Lord Zephyle could not... Lord Zephyle, of all people..."
Grimm murmured deeply, as if lost in reverie. For him, Zephyle was solely an object of admiration—someone he had probably never even considered catching up to.
Following after Grimm as he resumed moving, Rei spoke up.
"Trying to do what Zephyle couldn't isn't such a bad thing, is it?"
It wasn't just encouragement; Rei had some ulterior motives. If Grimm became able to freely travel to other worlds, then perhaps... truly perhaps, by some one-in-ten-thousand, one-in-a-hundred-million chance, Rei too might be able to visit Japan.
Of course, his current appearance was not that of Reiji Saiki from his time in Japan. His family and friends would not recognize him as Reiji Saiki even if they saw him now. Even so... even so, wanting to see his family and friends at least once more was only natural.
"When you put it that way, Rei, I somehow feel it might be possible. ...Of course, how things turn out with this current matter will make a significant difference. ...It has come into view."
Grimm, who had been advancing through the passage, stopped and murmured. At his words, Rei turned his gaze forward as well.
There, just as Grimm had said, the passage came to an end, and beyond it a faint light was visible. It clearly indicated that something lay ahead.
"...Now then, the question is what will appear. Whatever it is, it teleported that many things here, so it's undoubtedly a major player."
Inhaling to steady himself, Rei pressed forward through the passage, keeping his Misty Ring in mind so he could respond immediately no matter what happened.
Given that the passage was too narrow for Set to move through freely, it went without saying that Rei couldn't take out his weapons—Death Scythe and the Twilight Spear—in such a confined space. Of course, since he didn't know how spacious the area ahead was likely to be, there was a high probability he wouldn't be able to deploy them there either.
At any rate, Rei and Grimm did not proceed cautiously—they moved swiftly through the passage toward the light. Normally it would have been wiser to advance more carefully, but with both Grimm and Rei present, they could handle whatever came their way. That confidence was precisely what enabled their bold approach.
Upon emerging into the space, Rei and Grimm understood why it had been so bright.
"This is..."
The space was large enough to fit one noble's estate—perhaps even two. The ceiling was about as high as the descent through Grimm's passage, roughly ten meters or so. Near the center of this space, a mass of light... no, a mass of flame hovered.
It too was enormous—a towering mass of flame standing four to five meters tall, with a similar width.
"A Wisp, I presume."
"A Wisp? You mean, a Will-o'-the-Wisp?"
Will-o'-the-Wisp. An existence also called a ghost light or a Spirit, commonly known as a Wisp—just as Grimm had said. It was undoubtedly a monster, but while some were aggressive toward humans, others were friendly and would even come to their aid.
That said, as far as Rei knew—though only from books—Wisps were small, about the size of a fist, and even the largest were no bigger than a human head. It was by no means the size of what currently loomed before their eyes.
That was exactly why Rei couldn't immediately accept Grimm's assertion. But Grimm nodded in confirmation.
"Umu. From what I can see, the size is different, but otherwise it is undoubtedly a Wisp. ...However, the pale color of its flame concerns me."
"Pale?"
Rei looked at the Wisp again, but since this was his first time seeing one, he couldn't determine whether its color was pale or not. Grimm, unlike Rei, was a being who had lived for an extraordinarily long time—though Rei wasn't certain that was the correct way to phrase it. Naturally, he possessed deeper knowledge than Rei and understood a great deal about monsters. That was why Rei had no choice but to accept Grimm's judgment.
"Indeed. Of course, since there are various types of Wisps, the color of their flames also differs from individual to individual. Blue, red, yellow, white, and various others."
Told this, Rei once again turned his gaze toward the giant Wisp. The color was close to yellow, but if asked whether this was pale, his instinct was to question it—is it really?
"Setting the color aside, why is a Wisp... and such a massive one at that, here?"
"Given that it is here, I believe there is only one reason."
The meaning behind Grimm's words was clear. Why had they come here in the first place? Thinking of that, what the Wisp had been doing became obvious. The reason Rei didn't arrive at that conclusion immediately was likely because, based on his knowledge, he had never imagined Wisps possessed such abilities—something like teleportation between different worlds, of all things.
With the puzzle pieces aligned this far, though, Rei too reached the answer.
"Rare Species."
"Umu. Or perhaps a High-ranking Species, but."
Grimm affirmed Rei's words readily. Hearing this, Rei once again turned his gaze toward the Wisp. Despite Rei and Grimm being right there, it showed no sign of attacking—it simply drifted in the air.
Because Grimm is here?
Even if it were a Rare Species or High-ranking Species Wisp, faced with an extraordinary existence like Grimm, there was no way it could win. Perhaps it sensed that and couldn't bring itself to act.
That was what Rei thought, but noticing him glancing back and forth between Grimm and the Wisp, Grimm shook his skull.
"It is not because I am here that it does not move. It also relates to what I mentioned earlier about the pale color—most likely... it has exhausted its magic power to the extreme from repeatedly performing teleportations from another world."
Hearing that, Rei had no choice but to agree. If it had only teleported Lizardmen and Green People, that would be one thing, but it had also teleported the Birth Tower and a massive lake. Considering that, even someone with as vast a magic power as Rei might question whether it would have been enough.
Thinking that... then why did it teleport Lizardmen again today? That was the question that lingered in Rei's mind.