It was just before the final battle.  Kagaya planned to blow up the Hashira meeting place after luring Muzan in. where lady Tamayo would then poison Muzan.

Kagaya: “Yes, I’m aware your soul Is now ‘Devine’”

Muzan: “I Don’t care to toil with you broken humans..”

Kagaya: “Then dont toil with the corps, and prepare for war.”

Muzan: “You think you can kill me?”

Kagaya: “Smile thank you, Muzan.”

Muzans eyes widen as a light appears behind them, Muzan now realized what was happening