The Butler, Fascinated (その執事、感興, Sono Shitsuji, Kankyō) is Chapter 97 of the Kuroshitsuji manga.
After Sebastian correctly states that it is the Emerald Witch's duty to create the magic circle, Sieglinde explains that werewolves are clad in miasma, which is an evil magic that harms humans; a long time ago, miasma was present in the atmosphere in great amounts, and proved to be harmful to humans but necessary for magical beasts; when the humans prospered, the magic was lost, and the witches and werewolves lost their power as well; the werewolves then told the witches to create the source of miasma in exchange for their protection of the Werewolves' Forest; thus, the witches before Sieglinde researched extensively on the formula to create miasma, and such formula is given by Sieglinde to the werewolves to activate the Ultimate Spell that is able to endlessly create miasma with tremendous density; additionally, the miasma in the Werewolves' Forest are dregs of the ancient formulas.
Sebastian comments that that is the history Sieglinde was taught, and moves the altar in the center of the circle to unveil a hidden lever. He pulls the lever, which reveals a secret passageway behind a wall, much to Sieglinde's surprise. Sebastian calls it the entrance to the real "outside world," and she firmly declares that she will not turn back. They enter the chamber, and use an elevator to descend to floor B4. They soon discover a room filled with radars, gadgets, and monitors that showcase the map of Wolfsschlucht and track the current positions of the villagers via the amulets that act as transmitters sending a certain signal. Sieglinde exclaims in disbelief, but is silenced by Sebastian. They maneuver to another door, which leads to a large and elaborate factory that specializes in the manufacture of chemicals.
Subsequently, they hear noise and find the village crone holding an assembly with werewolves. The village crone proclaims that Sieglinde has finally done her job, and says that the miasma, which is condensed in a tiny bottle, is going to change history.
Sieglinde impulsively demands an explanation. The werewolves rush to her out of concern. Sebastian slices a werewolf costume, exposing a man inside it in the process, and remarks that the man had a mask inside the costume's nose. Ciel says that the "werewolves" were simply men in costumes, and the miasma is a chemical weapon constructed in the factory—it is fundamentally poisonous gas. Following Ciel's disclosure that the werewolves and the curse were all lies told to deceive Sieglinde, Sieglinde becomes overwhelmed with shock. The village crone admits that what Sieglinde performed was not magic; she maintains that, on the contrary, Sieglinde made "the most powerful poisonous gas in history," and decides to name the said gas "SuLIN."