Itachi's connection to Shikkotsu Forest began two years earlier, when he happened to rescue a baby slug named Miaomiao from a snake's jaws. Miaomiao was the child of Shikkotsu Forest's founding sage, the Slug Sage whose true name was Katsuyu.
Although she had a child, Katsuyu had no mate. This miraculous phenomenon stemmed from her little-known ability of self-breeding, which she possessed in addition to her ability to divide herself.
Itachi visited Shikkotsu Forest to take Miaomiao home.
But Katsuyu, who had lived for over a thousand years, did not readily trust a human like Itachi. She believed he was just like the visitors before him, coveting nothing but Shikkotsu Forest's power of Senjutsu!
Still, Itachi had brought her child back. Rather than kill him outright, Katsuyu devised a trial within an illusion.
Itachi passed that trial. During it, Katsuyu also used the Sage Art Mind-Reading to learn about his past, including the truth behind his slaughter of the Uchiha Clan and the heavy burden of his long-held wish. What she discovered moved her deeply!
And so, Itachi and Katsuyu became friends despite the vast difference in their ages.
Itachi promised not to reveal his visit to Shikkotsu Forest, and Katsuyu likewise agreed not to disclose the truth about him to those involved. This was why, in his previous life, the outside world had never learned of his connection to Shikkotsu Forest.
It was also why, when Itachi, revived through Impure World Reincarnation, fought alongside Sasuke against Kabuto in Sage Mode, he had blurted out the names "Shikkotsu Forest" and "Ryūchi Cave."
Back then, Katsuyu had noticed that Itachi suffered from an illness incurable by human means and offered to let him stay in Shikkotsu Forest to recover. But Itachi had other intentions. He had promised the Third Hokage that