A Light Novel prequel to Fate/Stay Night written by Urobuchi Gen with art by Takeuchi Takashi, later adapted into an anime by studio Ufotable.
Set 10 years ago from Fate/Stay Night, it tells the story of the Fourth Holy Grail War - as it counts down to the Zero Hour of the Fuyuki Fire.
Unlike the two Visual Novels there is currently no known or known planned official translation.
However, Baka-Tsukai have had a translation out since 2014.