The reason was that Seiichi had activated his Eternal Mangekyō Sharingan again right in front of the Nine-Tails.
Others might not be able to tell the difference between the Mangekyō and the Eternal Mangekyō Sharingan, but the Nine-Tails knew it perfectly well.
The difference in Eye Power between the Mangekyō and the Eternal Mangekyō Sharingan was so vast that they were worlds apart.
"The same eyes as Madara Uchiha!!!"
"Yeah, plenty of people have seen them, but you're the only one who recognized them at a glance and was absolutely certain. Impressive, impressive!"
Seiichi gave the Nine-Tails a thumbs-up.
You did not hit a smiling face. Starting with a few kind words would make the conversation that followed easier, wouldn't it?
After all the beatings the Nine-Tails had taken over the years, it was nowhere near as stubborn as it had been at first.
"Go on, then. Why have you come today? Don't tell me you want me to stay quiet and not cause trouble."
"Of course... not!"
Seiichi drew out his words, then changed course when he saw the veins bulging on the Nine-Tails' forehead, indicating that it had guessed wrong.
"We're here today to let you out. We just wanted to tell you not to cause trouble in the meantime. Let her deliver her baby peacefully and safely, and we'll let you out immediately afterward."
"Hmph, you humans are so fucking hypocritical!" The Nine-Tails panted heavily, glaring at Seiichi and Kushina.
Were they insulting its intelligence by saying that?
"I'm not lying. If we didn't want to release you, I could just take control of you. You know I can do it, and very easily, too. So there's really no reason for me to waste time lying to you like this, is there?"
Seiichi spread his hands helplessly