But, the Holy Grail did that impossibility render as possible.
Summoning whole Heroic Spirits originally beyond the control of man, and thereon affixing them as Familiars in the service of Masters.
Such nonsense was verily the proof that the Holy Grail was omnipotent (万能, bannou, "of ten thousand potencies").
And irrelevant of era — from as near (近く, chikaku) as a hundred years prior (百年前, hyakunen-mae); or as far as the vicinity of the Age of Divinities (神代の頃, jindai no koro) — Heroic Spirits have been summoned forth.
Seven Heroic Spirits serve seven masters — protecting their own Master, and expelling those Masters that are their enemies.
... Heroic Spirits of every era; every nation do in modernity revive, to vie for supremacy in a deathmatch.
Such would be the reason that this ritual is called as the War for the Holy Grail, perhaps..