Once again a secret war begins. This time Shinjuku, Tokyo, will become the battleground for a new Heaven's Feel. Fourteen mages from across the world gather with their Servants, Heroic Spirits called forth from the memory of the world from legends and stories of old, in order to battle for the greatest prize imaginable.
The Holy Grail.
Said to be able to grant any wish imaginable to the victor of the Grail war, the fourteen Masters will fight one another with their Servants to the best of their strength, cunning, and will. But only one master and servant pair will stand above the rest and claim the Grail for their own.
The Holy Grail War is divided into two factions; the Red Faction, and the Black Faction. Both are currently being manipulated by unseen figures for unknown goals, but the means to summon Servants in this Grail War originate from them, one way or another-- perhaps the artifact you used was placed there to be found by them, or perhaps they provided direct help.
The Servants So Far
Saber of Red: Mordred, Servant of Elliana Fairchild
Saber of Black: Jeanne d'Arc, Servant of Shizune Nakamura
Assassin of Black: Jack the Ripper, Servant of Himei Shoutan
Berserker of Black: Kiyohime, Servant of Kay
Lancer of Black: Vlad III, Servant of (Black Faction NPC).
Though at a glance it would appear the Red Faction favors of the Confederacy, and the Black Faction favors of the Union, neither has actually made a move to contact the superfactions or even expose themselves at all, and it may simply be a coincidence.
Update: the Black Faction has officially sent an envoy to the Union, with their Servant, Lancer of Black.