The Lord El-Melloi II Case Files novel series is a Fate/ spin-off featuring the mage mystery solving efforts of Lord El-Melloi II (aka the adult-aged Waver Velvet) as told from the point of view of his teenage apprentice Gray.
Though set in an alternate world, Waver still participated in the Fourth Holy Grail War as seen in Fate/Zero.
Well timed for Waver’s October 3rd birthday, an eBook version of the novels written by Sanda Makoto (Rental Magica) and illustrated by Mineji Sakamoto were released in Japan, and, the new November issue of Young Ace magazine launched a manga adaptation by TENGEN.