The Butler, Sage is Chapter 131 of the Kuroshitsuji manga.
When Diedrich visits, "Ciel" sits with Vincent, while Ciel hides behind the curtain. Vincent asks Diedrich to look after his kids for him if anything happens to him. Diedrich tells Ciel that since he is the younger son, he will not inherit the title or the land, so he should at least be brave like his older brother.
Ciel runs off. He envies "Ciel" for being able to talk to anyone, recalling the incident when "Ciel" greeted Kelvin at a party the other day.
After Ciel bumps into Undertaker, "Ciel" catches up to him. Pitt arrives, shows off the new camera he got from reporting on one of Vincent's closed cases, and offers to take a picture of the twins. Undertaker holds up a cloth while the twins pose, and Pitt snaps a picture of them.
Later, "Ciel" gives Ciel the food he, Vincent, and Diedrich made for him. When Ciel tells "Ciel" he can go out and play without him, "Ciel" assures him that he is with him because he wants to be. At night, "Ciel" prepares Ciel warm milk with honey.
The next day, Ciel is unable to go boating because of his cough. He urges "Ciel," Vincent, and Rachel to go on without him. Tanaka acknowledges his sacrifice, and allows him to play chess and eat anything he wants.
Eventually, "Ciel," Vincent, and Rachel return with bundles of flowers for Ciel. "Ciel" trips while attempting to deliver the flowers, and they all laugh. Ciel thinks to himself that he was happy until "that day" came.