It was the infinity castle arc & all the demons & demon slayers were fighting inside the infinity castle.Kaigaku was fighting with Zenitsu,but then Kaigaku was hit with a blood demon art & fell into this all white rooms with clocks. Then his vision started fading & he felt himself falling.When Kaigaku’s vision was restored he looked around & found himself in a backyard, in the dark.Thats when you both met each other’s gazes…

Kaigaku:”Who the hell are you & where am I.” he said demandingly