Ichikishima Hime was the most human-like sage among the Ryūchi Cave hierarchy, though she rarely walked, preferring to float in the air. This drew crowds of onlookers, for in their minds, no human could fly. Yet Ichikishima Hime paid no heed, curious about everything in the human world. She bought countless items—all with Seisuke's money, of course.
After accompanying Ichikishima Hime until evening, having dinner, and returning to his home, she stretched and said, "Hmm, the human world is indeed more interesting. It's given me so much inspiration."
"Inspiration? What kind of inspiration?" Seisuke asked curiously.
"Of course, inspiration on how to trick humans. We failed to trick you last time and realized we might be out of touch with the times. You humans have developed incredibly rapidly in recent years, and since few visit Ryūchi Cave, our trials have become too easy."
Ah, so their trials were like child's play. But out of politeness, he didn't voice this, only mourning for future trainees—fortunately, it wasn't him.
He then asked, "By the way, can I start learning how to enter sage mode now?"
"Sure, but the natural energy here is too thin. Let's go back to Ryūchi Cave. Remember to bring all my things."
"Got it, got it." Seisuke sealed all the items Ichikishima Hime had bought into a sealing scroll, then headed to Ryūchi Cave with her.
Lady White Snake Sage still coiled on the stone table, her massive body merging with Ryūchi Cave itself. Seeing Seisuke arrive, she didn't speak. Instead, Tazunohime approached and said, "Asakawa Seisuke, you've come to learn senjutsu, right?"
"Yes, but I still can't enter sage mode."
Tazunohime replied, "I'll guide your training next. Follow us."
"Okay."
Trailing behind Tatenokami Hime, Tazunohime, and the others, they entered a pitch-black cave. Flames ignited, illuminating the cave, revealing a stone platform at its center. Tazunohime pointed to it: "Sit there."
"Alright."
Cross-legged on the platform, Tazunohime said, "Focus on the trace of natural energy Lady White Snake Sage left in your body."
"Um, I won't turn into a snake this time, will I?" Recalling the previous pain, he felt a twinge of fear.
Tazunohime shook her head: "No. In fact, learning Ryūchi Cave senjutsu usually turns humans into snakes, but losing consciousness or failing during the process means failure. Failing to master senjutsu in any of the Three Great Sannin Lands turns humans into the form of that land's inhabitants."
Just as failing at Mount Myōboku turns one into a stone toad, Ryūchi Cave accelerates this failure. The transformation is excruciatingly painful, with the risk of death at any moment—far more brutal than senjutsu training at Myōboku or Shikkotsurin, yet paradoxically, the entry barrier is lower. At least, that's how it seems.
With Lady White Snake Sage's natural energy already in him, Seisuke didn't need to learn how to sense natural energy. Triggering that trace would automatically absorb it, eventually becoming second nature—his body would absorb natural energy instinctively when he wished to use it.
Closing his eyes, Seisuke concentrated, carefully sensing the unusual energy within. His mind quieted, and gradually, he detected that special force.
Tazunohime's voice echoed: "Once you sense it, use your chakra to trigger that natural energy."
Following her instructions, Seisuke immediately used his chakra to touch the natural energy. The moment they made contact, the natural energy surged like a domineering CEO absorbing a delicate wife, overwhelming his chakra, then furiously extending outward to merge with it, forming senjutsu chakra. Simultaneously, his perception of natural energy sharpened, drawing more into his body to accelerate the fusion.
As they merged, the surrounding scenery became "clearer" in Seisuke's mind. He sensed the positions and distances of Tazunohime and the other sages. The cave's layout materialized in his mind, and his perception expanded, detecting more and more snakes.
Around his eyes, a purple sage face pattern appeared, resembling Orochimaru's eye shadow extending to the sides of his nose, adding an indescribable charm to his handsome features.
"No snake transformation? A perfect sage mode," Tatenokami Hime remarked. An imperfect sage mode would make humans resemble animals—Jiraiya's flawed mode, for example, made him look frog-like, while Kabuto grew horns. (Of course, the snake on Kabuto's stomach likely stemmed from his own modifications, not Ryūchi Cave.)
Seisuke opened his eyes. His round pupils had turned golden, snake-like slits—reminiscent of a younger Orochimaru.
"Is this... sage mode?"
Now, his body brimmed with senjutsu chakra, doubling his strength and then some.
"Wait, what's this?" He felt a foreign object in his eyes. Blinking, a transparent membrane descended, covering his eyes.
"That's a cornea," Ichikishima Hime explained. "Even though your sage mode is perfect, the cornea comes with it. Now I'll teach you heat sensing."
Snakes possess heat-sensing systems to detect prey via body heat (since they're highly myopic). While Ryūchi Cave snakes have good vision, heat sensing is still useful in darkness.
"Got it."
This would be invaluable on the Mist Ninja battlefield, where enemies often hid in thick fog. Heat sensing was tailor-made for such situations.
Learning heat sensing wasn't difficult for Seisuke. Though he lacked this ability originally, his body had undergone subtle changes after transforming into a snake—including developing a heat-sensing system. Ichikishima Hime simply used senjutsu to activate it, a capability he could use even outside sage mode.
Heat sensing was 360-degree. Before his "eyes," three thermal images of varying colors appeared.
"This is so strange." The sensation was odd. Ichikishima Hime's "teaching" focused on helping him adapt to his own thermal vision.
"Alright, next comes senjutsu. We have several types here. I'll list them—you can learn three," Tazunohime said.
Here it comes—senjutsu!