The Butler, Asserting is Chapter 140 of the Kuroshitsuji manga.
In 1886, Ciel declares that the demon is now his "dog" before turning to the corpse of his brother and apologizing to him that he has to break their promise to each other as he cannot take him with him. Afterward, Ciel commands the demon to burn down the cathedral and the bodies. The demon praises Ciel for his decision as it makes the lie more "complete." Then, he proceeds to burn everything down.
Ciel takes one last look at his twin brother "Ciel" before heading out, the cathedral going up in flames behind him.
In 1889, "Ciel" starts to laugh and says that Ciel does not have to look like his "about to burst into tears." Then, he says that he will not punish Ciel for his lies, but everyone who dares to call him a liar. "Ciel" proceeds to praise Ciel for having done his best for the past three years and adds that he does not have to lie anymore as he is finally home.
Ciel, still shocked at his brother's return, can only stammer a reply while Sebastian declares that he is "quite certain ["Ciel"] lost [his] life that night." He is then cut off by Undertaker who appears at the top of the stairs with an anatomical model of a human.
Immediately, Ciel asks if Undertaker was the one who took "Ciel", but Undertaker interrupts him, saying that he needs his payment first—however, the whole "Starlight Four/Phantom Five" affair had amused him to no end and made him laugh enough "for three years," so he admits that he saved "Ciel" from the fire which was quite the ordeal—an ordeal he gladly took because he cannot resurrect "Ciel" when he is burned to ashes and did not want to lose another Phantomhive.