The relationship between Itachi and Shikkotsu Forest began when he accidentally rescued Katsuyu's baby 'Miaomiao' from Shekou two years ago. Miaomiao is the child of Slug Sage, the founder of Shikkotsu Forest, whose real name is 'Katsuyu'.
Although he is a child, Katsuyu has no partner. This magical phenomenon comes from the fact that Katsuyu has the little-known ability of 'Self-Breeding' on top of being divided.
To send Miaomiao home, Itachi visited Shikkotsu Forest.
However, Katsuyu, who has lived for thousands of years, does not easily believe in Itachi, a human being, and thinks that he is just like those visitors in the past, just coveting Shikkotsu Forest's Senjutsu power!
But Itachi finally brought her child back. Katsuyu did not kill Itachi directly, but set up an illusion test.
Itachi passed that test. During the test, Katsuyu also learned about Itachi's life history through Sage Art of 'Mind-Reading', including the truth about Itachi's killing of the Uchiha Clan, and the heavy long-cherished wish he carried, which greatly touched him!
In this way, Itachi and Katsuyu became close friends.
Itachi promised not to reveal to the outside world that he had visited Shikkotsu Forest, and Katsuyu also stated that he would not reveal the truth about Itachi to relevant personnel. This is also the reason why the relationship between Itachi and Shikkotsu Forest in his previous life has never been known to the outside world.
This is also the reason why Impure World Reincarnation's Itachi and Sasuke blurted out "Shikkotsu Forest" and "Ryūchi Cave" when they teamed up to fight 'Sage Mode Kabuto'.
When Katsuyu noticed that Itachi was suffering from a disease that was incurable for humans, he proposed to stay at Shikkotsu Forest to recuperate, but Itachi had no intention of doing so. He had the mission promised to Third Hokage, to monitor Akatsuki as an undercover agent, and was determined to die in the battle with Sasuke, so that Sasuke could inherit his eyes and gain eternal light.
After expressing his determination, Katsuyu chose to respect him and gave him medicine that could delay the onset of the disease, as well as a potion that could delay Mangekyo's blindness!
This is also the reason why the brightness of Itachi's eyes lasted until the battle with Sasuke without any obvious decline!
However, even though Itachi had a close relationship with Katsuyu, he never learned Senjutsu from Shikkotsu Forest!
Under the guidance of Miaomiao, we came to the main body of Katsuyu.
Although we have always known each other, Itachi did not forget the courtesy of the junior, and saluted from the Tuxia seat. "Sir Katsuyu, junior Uchiha Itachi came here unexpectedly, I am very sorry!"
The body of Katsuyu is bigger than 'Susanoo Complete Body'. When you look up, you can only see a patch of white flowers!
It was quite inconvenient to communicate like this. Katsuyu used a similar ability to Ryūchi Cave White Snake Sage to transform himself into the figure of a human girl. He landed gracefully in front of Itachi. After lifting him up, he said with a sweet smile, "Itachi, I'm very happy that you can come."
Itachi smiled and nodded.
After taking a seat.
Katsuyu: "Why has your condition worsened again? Have you been too worried recently?"
Itachi: "To tell you the truth, something happened to my brother that is beyond my understanding."
Katsuyu: "Sasuke, it must be Orochimaru! Next time we meet, I will melt it with Acid!"
Itachi's eyes were downcast. "The problem probably isn't just Orochimaru..."
Katsuyu: "Can you tell me in detail?"
Itachi told Katsuyu about the series of actions Sasuke had launched against Akatsuki since he defected from Konoha, and... his horrific record!
Katsuyu fell into deep thought after hearing this, and then became confused. "I also don't understand. If he can be brought to me, I might be able to use 'Mind-Reading' to know the truth of the matter, but that doesn't seem realistic..."
Itachi said, "So, Sasuke and I are done with it in advance. I have already sent out an invitation, just one month from now."
Katsuyu: "One month? It is indeed too stressful. And from your description, his power now seems to be greater than yours."
Itachi said, "There is nothing wrong with dying at the hands of Sasuke, but at least I want to figure out what happened to Sasuke through this battle."
Katsuyu: "Itachi, just tell me, is there anything you need my help with?"
Itachi said sincerely, "Please teach me the power of Senjutsu!"
Katsuyu had already guessed Itachi's intention, so he was not surprised.
She stood up and walked around, turning her back to Itachi.
"The essence of Senjutsu is the balance created by physical energy, mental energy, and Natural Energy. The methods of Three Sage Lands are different due to their different emphasis."
"Mount Myōboku focuses on 'Natural Energy' and is also the most balanced among the three Senjutsu, so it is called 'Harmonizing Sage Art'."
"Ryūchi Cave focuses on 'body energy'. When activated, the body's representation will change significantly, and the mood will become extremely excited, so it is called - 'Transformation Sage Art'."
"My Shikkotsu Forest's Senjutsu is the opposite of Ryūchi Cave. It focuses on spiritual energy, especially the control of the 'heart'. When activated, the spiritual energy will be raised to a new level and will enter the state of mirror Shisui, so it is called - 'Stillness Sage Art'!"
Katsuyu turned around.
"Itachi, as you were born in the Uchiha Clan, you have abundant mental energy and a transcendent state of mind, so you have the conditions to learn Stillness Sage Art. However, Senjutsu is an art of 'balance' after all, and physical energy is your shortcoming. Therefore, you cannot fully master Stillness Sage Art like 'Mito' did, but you can still master half of it..."
Itachi: "Of course, I don't expect to achieve the achievements of Lord 'Mito Uzumaki'. Being able to master half of the power of Senjutsu is already extremely extraordinary to me!"
Katsuyu: "Come with me, Itachi, I will take you to 'Stillness Forest' and perform the initiation ceremony of Stillness Sage Power there. Then, you will have to rely on yourself!"
"Understood!"
Itachi has decided that if the many mysteries lingering on Sasuke cannot be solved through all-out fighting, then he will use the 'Left Eye' left by Shisui to solve the problem of Sasuke activates the strongest Genjutsu--"Kotoamatsukami" to prevent Sasuke from being used by Orochimaru to do things that endanger Sasuke itself!
Although this method... he was extremely resistant in his heart, but by working as an undercover agent in Akatsuki... Itachi knew very well how deep Shinobi World's dark side was. Although Sasuke showed his performance to surpass him, Itachi was really afraid that Sasuke would make a careless move and be doomed!
With all the arrangements in place, it was ensured that Sasuke would live as he expected after his death, but Itachi, deep down in his heart, was paradoxically expecting... Sasuke to completely break his expectations!
He was monologued in his mind.
--"Everyone relies on their own knowledge and cognition, but lives bound by it, and calls it reality. However, knowledge and cognition are ambiguous things, and reality may be just a mirror. Suigetsu. People live in self-awareness... Sasuke, can you break my "reality"? That way, even if I die, I will be proud of you!"
In this way, Itachi and Sasuke, with their thoughts and beliefs that they had never had in their previous lives, passed in a blink of an eye!