The phrase "ago no line" is a pun in Japanese, referring to the shape of Abberline and Hank's chins and a play on Abberline's name. This would roughly translate to "ass face and sharp" in English.
Yuya Matsushita, who played as Sebastian Michaelis, wrote and performed a single pointed to 'The Most Beautiful Death', titled "YOU". The single was sold at the musical preview as a promotion for the show. It was later included as a track in his album "I am Me" along with an extended and instrumental versions of "Hallucination".
The writer of 'Kuroshitsuji' (Black Butler) designed the character's clothing specially for the show.
Eric and Alan's death scene is later seen in chapter 105 of 'Kuroshitsuji', making the events of this show a cannon part in the 'Kuroshitsuji' universe.
Abberline and Hanks' opera dresses were designed by the San Francisco Lolita dress company, Alice and the Pirates.