In the Nine-Tails' memories, the last time it had seen the old Sage was at the end of the Fourth Shinobi World War, when he had manifested through half of Madara Uchiha's body after Madara had attained Six Paths power.
Back then, the old Sage had indeed retained his chakra to protect and watch over the Shinobi World and the Pure Land.
After the old Sage disappeared again, even the Nine-Tails had no idea whether he would ever return.
"Old man? It really is you..." The Nine-Tails was delighted at first when it saw him, but after looking around, its joy quickly faded. It asked gravely, "No, what's going on here? Where's Naruto? He came here with me. Where is he?"
Yet the Sage of Six Paths, floating in the air, was also frowning. "...Why are you and Naruto the ones who came? Shouldn't it have been the God of Ōtsutsuki?"
His words stunned the Nine-Tails even more. It hadn't expected to hear the name God of Ōtsutsuki here. "What do you mean?"
"I'm a portion of chakra separated from my original body. This is a trap set for the God of Ōtsutsuki. How did you two trigger it?"
"...Wait, let's start from the beginning. To put it simply, it was because of that tanuki Shukaku..."
To the Nine-Tails, the old Sage was the only elder it acknowledged as family. It hid nothing and explained the situation to him.
"...So that was what happened?"
After hearing the Nine-Tails' account, the Sage of Six Paths was filled with emotion. "Reborn within Naruto in the future... Through chance and circumstance, fate actually took such a turn."
"I had put safeguards in place to keep later generations from entering this trap, but I never accounted for your presence... This trap was originally a