Back during the Nine-Tails attack on Konoha, after his battle with Obito, Namikaze Minato went to seal the Nine-Tails. Almost the entire process was witnessed by Black Zetsu, who had been hiding in the shadows.
Naturally, Black Zetsu was well aware that sealed inside Namikaze Minato was the chakra of the other half—Yin Kurama!
Plotting to get his hands on that half of Yin Kurama, Black Zetsu kept his guard up against a sudden attack from Uzumaki Naruto while secretly transmitting his voice to Kaguya:
Chapter 234