Fate/Zero, a 4-volume light novel series, is a prequel to Fate/stay night. The novels were announced as a collaboration project between TYPE-MOON and Nitro+, written by Nitro+'s Urobuchi Gen and illustrated by TYPE-MOON's Takashi Takeuchi. The first volume was released during Comiket 71 in December 2006, and the remaining three volumes release in 2007.
Due to its success, the novel series was later adapted into a 2-season anime series by ufotable. The first season aired in 2011 winter, followed by the second season in spring 2012.
As its name implies, Fate/Zero's story, centered around the 4th Holy Grail War, takes place 10 years before the events of Fate/stay night, and involves family members of characters from the 5th Holy Grail War, as well as some familiar faces from the original game.