The Grail itself also summons Ruler to act as a mediator of the Holy Grail War.
Plot concept: The setting of Fate/Apocrypha is a parallel world to Fate/stay night where the Greater Grail was removed from Fuyuki after the Third Holy Grail War and disappeared for many years.
The one remaining manages to activate the reserve system of the Greater Grail, allowing for the summoning of fourteen Servants in total.
The Association dispatches fifty magi to retrieve it, but all but one are instantly slaughtered by Lancer of Black.