The Holy Grail War is a concept that appears in the Fate series. The battle surrounding the "Holy Grail" that can fulfill the wish of the holder is the Holy Grail War in a broad sense.
About once every sixty years, the spiritual power in the ley lines of Fuyuki City will accumulate enough to support the birth of the Holy Grail, and then the Holy Grail, which has the power to instantly realize any wish, will appear in Fuyuki City.
However, only a group of Masters and Servants can obtain this power. Therefore, an unwritten covenant was made, and the seven magicians, leading the heroic spirits they summoned, fought for the ownership of the Holy Grail. The winner who survived in the end would obtain the ownership of the Holy Grail - this was the Holy Grail War in Fuyuki City.