Kosu followed the Guild Master, Tetsuo through the winding corridors of the Adventurer's Guild until they reached a hidden entrance. Tetsuo pushed aside a large boulder, revealing a small opening that led to a steep descent down a narrow stone stairway. As they descended, Kosu could hear the sound of running water growing louder.

Finally, they emerged into a large cavern illuminated by glowing blue rocks. The walls, ceiling, and floor were all made of the same blue stone, and Kosu could see a clear stream running through the middle of the cavern.

"This is our secret training lair," Tetsuo said with a hint of pride in his voice. "The blue stone here is infused with magic, making it an ideal environment for training."

Kosu looked around in wonder, taking in the stunning beauty of the underground cavern. "It's amazing," he said.

Tetsuo nodded. "But it's not just the environment that makes this place special. It's also the training methods we use here. And I'm going to teach you one of the most powerful techniques we have."

Kosu's eyes widened in excitement as Tetsuo led him to a secluded corner of the cavern.

Tetsuo sat across from Kosu, his expression serious. "You've likely noticed by now that you can't use domain magic, despite your formidable strength," he said. "That's because the power you possess comes from a different source."

Kosu raised an eyebrow. "A different source? What do you mean?"

"The power you have, Soul Charge, comes from within yourself," Tetsuo explained. "It's not tied to any specific element or domain. That's why you can use it even in places where domain magic is restricted, like dungeons. And because it's tied to your own internal energy, you can continue to use it even when you're exhausted, as long as you have the willpower to keep going."

Kosu leaned back, absorbing this information. "So that's why I've been able to take on so many strong opponents in the dungeon," he mused. "But if I can't use domain magic, doesn't that mean I'll never be as powerful as someone who can?"

Tetsuo shook his head. "Not necessarily. Soul Charge is a powerful skill in its own right, and it has advantages that domain magic doesn't. With the right training and focus, you can become just as strong as any domain mage, if not stronger."

Kosu nodded, feeling a newfound sense of purpose. He had always known he was strong, but now he had a better understanding of why. And with Tetsuo's help, he was determined to master his power and become the strongest adventurer he could be.

Tetsuo explained to Kosu that the core principle of Soul Charge is all about the user's control over the flow of magic particles. Instead of letting the magic particles flow out of their body like a typical magic user, the user of Soul Charge must absorb magic particles from their surroundings through their mana gate, and then send it to every part of their body through the acupuncture points. This process required precise control over the mana gate and acupuncture points to prevent an overload of magic particles, which could be dangerous to the user's body.

Tetsuo began to teach Kosu about the acupuncture points in the body. "Acupuncture points are the key to unlocking the potential of the Soul Charge," Tetsuo explained. "There are hundreds of them, but the most important ones are the ones along the meridians. They are like channels through which the energy of the Soul Charge flows."

Kosu listened intently, trying to absorb all the information. Tetsuo continued, "To open the acupuncture points, you must use your internal energy to stimulate them. It's like unlocking a door with a key. Once the point is open, the energy will flow freely, and you will feel a surge of power."

Kosu asked, "But what if I can't open all the acupuncture points?"

Tetsuo replied, "It's not necessary to open them all. Even if you can only open a few, it will make a vast difference in your strength. The important thing is to focus on the ones that will enhance your abilities the most."

Kosu nodded, understanding the importance of opening the acupuncture points. 

Tetsuo continue, shifting his focus to the mana gates in the body. "Mana gates," he said, "are the entry and exit points of magic power in the body. They act like valves that control the flow of magic power in and out of the body. Opening and closing them is essential to using Soul Charge."

Kosu listened intently as Tetsuo demonstrated on his own body, pointing out the various locations of the mana gates and how to manipulate them. He explained that the gates could be opened partially or fully, depending on the user's skill and level of control.

"As you practice opening your gates, you'll feel a rush of magic power flowing through your body," Tetsuo said. "This power can be overwhelming at first, but with time and practice, you'll learn to control it and harness it for your battles in the dungeon."

Tetsuo demonstrated this by showing Kosu how to open his mana gate and control the flow of magic particles, gradually increasing the amount that he could absorb and send to his acupuncture points. Kosu found it difficult to open every acupuncture point in his body, but even with a small number of open points, he could feel the immense power of Soul Charge coursing through him.

Tetsuo explained to Kosu that his lineage possessed a unique body type that allowed them to handle the extreme power generated by controlling their acupuncture points and mana gate. He revealed that this was a trait that had been passed down through generations and that Kosu was fortunate enough to inherit it. Tetsuo further elaborated that while normal people could learn to control their acupuncture points and mana gate, their bodies would not be able to handle the extreme power generated by the Soul Charge skill, resulting in severe injury or death..

"The ability to absorb magic particles from the surrounding and channel it through your acupuncture points and mana gate is what makes Soul Charge so powerful," Tetsuo said. "And your body's capacity to hold and handle that power is what makes you capable of performing the skill at near-limitless levels."

Kosu was amazed by the explanation and couldn't believe how fortunate he was to have inherited such a unique body type. He was eager to learn more about how to control his acupuncture points and mana gate, and how to maximize the potential of his Soul Charge skill.