The novels are set between the events of Fate/Zero and Fate/stay night, focusing on Lord El-Melloi II, a former master who now investigates several mysteries after his participation in the Fourth Holy Grail War.

A sequel novel series, The Adventures of Lord El-Melloi II (ロード・エルメロイII世の冒険, Rōdo Erumeroi Nisei no Bōken), began publishing on December 25, 2020.

It was then on TYPE-MOON FEST in July 2012 that Nasu asked Makoto to write a story for Waver Velvet/Lord El-Melloi II, on which he agreed.

A sequel novel series titled The Adventures of Lord El-Melloi II (ロード・エルメロイII世の冒険, Rōdo Erumeroi Nisei no Bōken) was announced in December 2019.

[10][11] A manga adaptation by Makoto Sanda with illustrations from Tō Azuma began serialization in Kadokawa Shoten's Young Ace magazine on October 4, 2017.

An anime adaptation of the novels, which only adapts the Rail Zeppelin storyline under the title Lord El-Melloi II's Case Files {Rail Zeppelin} Grace note (ロード・エルメロイII世の事件簿 -魔眼蒐集列車 Grace note-, Rōdo Erumeroi Nisei no Jikenbo {Reru Zeperin} Gureisu Nōto) was announced at the Fate Project New Year's TV special on December 31, 2018, which also aired a special episode of the series.

[31] On October 3, 2020, which is Waver's birthday, Aniplex of America announced they will be releasing the series in a Blu-ray box set with a full English dub.

A stage play adaptation based on the novel's "Adra Castle Separation" chapter, began on December 14, 2019 and concluded on January 19, 2020.