Great Holy Grail War

To describe it in a single phrase, it is a distorted, exceptional version of the Fuyuki-style Holy Grail War. In the case where the battle between seven Servants doesn't start for an overly long period of time, the Holy Grail will make preparations to summon seven new Servants. However, that is an action that once again sucks up the Grail's stored mana that it gradually amassed over sixty years from the ley lines, and if done improperly, the ley lines will become completely dried up forever.
Due to the work of the only magus who managed to escape from Vlad III's hands, the Greater Grail activated the Great Holy Grail War system and, though normally it would have made the Holy Grail War enter a standby waiting period, it was instead made to immediately recognize the summoning of seven additional Servants to fight in a great war.
However, the Great Holy Grail War was within the scope of Darnic's predictions. Because even if he formed a new organization using the Greater Grail as a symbol, the Mage's Association would definitely seek to obstruct it. As such, he judged that it would be best if they had a showdown against the Mage's Association once with their full power—that is, their Servants. Darnic was also farsighted enough to include increasing reactionary forces within the Association by thoroughly beating them here and bringing them into his own faction.
If his plans had all gone well, the Association's forces would have been diminished and brought about an era similar to the turbulent Sengoku Era in Japan.
…Well, it all came to nothing due to Shirou-san and Rider of “Black,” though.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia