Lord El-Melloi II Sei no Jikenbo

is a Japanese light novel series written by Makoto Sanda and illustrated by Mineji Sakamoto. Officially part of the Fate series, the story is set between the events of Fate/Zero and Fate/Stay Night and the novels were serialized by Type-Moon under its publication imprint on December 30, 2014, completing to 10 volumes.

A manga adaptation by Makoto Sanda with illustrations from Tō Azuma began serialization in Kadokawa Shoten's Young Ace magazine on October 4, 2017 and an anime adaptation by Troyca, which adapts the Rail Zeppelin chapter, premiered on July 6, 2019.

Plot
Lord El-Melloi II, formerly known as Waver Velvet, a successor of Kayneth and former master in the Fourth Holy Grail War, is a professor under the ranks of the Mage's Association. In his years as a professor in the Clock Tower, he is acting as the temporary head of the El-Melloi household until a proper successor can take over. But when strange mysteries involving magic start happening around London, he teams up with a mysterious person and apprentice named Gray in investigating many mysteries surrounding them and most of all, the mysterious organization he's been working with.

Official

 * Official site (Japanese)
 * Official site (Type Moon) (Japanese)