The Butler, Negotiating is Chapter 137 of the Kuroshitsuji manga.
Ciel watches the summoning of a demon who asks who is the one that spat upon God. The cultists are overjoyed at successful summoning and try to claim the demon as their own, but it rejects them. Then, the demon spots Ciel and realises that he is the one it was seeking.
Ciel is scared and does not understand what is happening or why the demon says that he apparently called it. The demon tells him that Ciel did call for him in the moment he denied God and paid the price for the summoning. At first, Ciel does not understand what the demon means with "sacrifice" before it dawns on him that the demon must mean "Ciel".
The demon confirms that "Ciel"'s soul had been the sacrifice and picks up "Ciel"'s corpse while telling Ciel that "by the virtue of the 'sacrifice'... [Ciel's] wish... and [their] 'covenant', [it] will become [his] slave until the day [his] silly wish is granted and [it] can claim [his] soul."
Ciel replies that he did not sacrifice his twin brother, but the demon, appearing as a dog, tries to convince him that he did—after all, if he had not, it would not be here. Ciel still denies his apparent sin and demands "Ciel" back, but the demon, taking the forms of a raven, an octopus, and a goat, keeps telling him that Ciel was the one who summoned it, that it was the unshakeable truth, that what was lost could not be brought back. However, the demon also says that it is Ciel's decision whether or not he will form a Faustian contract with it.
Frightened and certain that nobody would be happy for the "spare" to return, Ciel is unsure what to do—would it be worth it to return if there was nobody to welcome him? If there was nothing to return to?
During his panicked pondering, Ciel recalls his brother's promise that he would always be there to protect him. Ciel finally understands that "Ciel" is gone forever, that he will never return—and with this knowledge and with the belief that he will never be anything else but a coward, but his brother's spare, Ciel decides that he will become the "Ciel" his brother had wanted to be and reaches out to the demon, ready to form a covenant in exchange for the power to take revenge.