The Holy Grail War.
The Holy Grail is a mystic object, and artefact of power that knowns unbound power. The specifics regarding the Holy Grail are unknown, but it is a fact that it is not the vessel that received the blood of God; its power rivals that of the original, however, being able to grant one wish - and one wish only - to its wielder.
To summon the Holy Grail, however, seven magi are necessary.
One wish, seven magi.
The Holy Grail War was born.
Seven magi, known as Masters, cooperate to summon the power of the Holy Grail to the earth, specifically at Fuyuki City, which due to its convergence of leylines is a suitable place. Then, they proceed to kill each other through the use of Servants, Heroic Spirits summoned forth by the power of the Grail and who wield power far surpassing that of humans.
That is the terror known as the Holy Grail War.