The Butler, Lamenting is Chapter 134 of the Kuroshitsuji manga.
Ciel now waits alone in their room, getting more and more scared, and when the grandfather's clock rings at seven o'clock, he hesitantly leaves the room. He runs through the manor's corridors and room, calling if anybody is there but gets no answer. When he hears a strange sound coming from one of the rooms, Ciel slowly goes to open the door, only to find Sebastian, muzzled, inside.
Ciel removes the muzzle from the dog and wants to proceed to search for "Ciel," but Sebastian holds him back and guides him in the opposite direction. Sebastian runs ahead, and Ciel has troubles catching up with him. Eventually, he loses sight of him but hears his barks coming from his parents' bedroom.
There, Ciel finds not only Sebastian's but Rachel's and Vincent's corpse. Horrified, he runs out of the room after seeing all the blood, frantically hurrying through the manor until he trips over another corpse. Screaming, Ciel stands up and continues to run through the mansion, only to find more and more corpses.
Eventually, he reaches the dining room and, upon seeing the destroyed birthday decorations, he breaks down crying. Ciel remains like that for a while until another noise makes him leave the dining room. In the corridor, he sees Tanaka and blindly runs towards him despite Tanaka telling him to stay away as what he would see if he kept coming closer is "far too awful for [him] to bear."
Then, Tanaka is stabbed in the back, and Ciel captured by an unknown person.