Aizen Mephisto Iori
"..."
Ciel stopped and looked at Iori, but Iori paid her no mind and continued strolling ahead, leaving the silver-haired girl with nothing but his back.
"I don't understand... Why are you trying to lead me on? How I wish upon the Holy Grail has nothing to do with you, does it?"
"So you still intend to devote all the Greater Holy Grail's power to fulfilling the Einzbern family's wish, rather than your own needs?"
Iori curled his lip. "What a stubborn girl. Have you never considered that you could have both?"
"The Greater Holy Grail's power must all be used to open the 'hole.' If mana is wasted and the hole cannot be opened, or is too small... then everything will have been for nothing!"
Ciel remained resolute, unmoved by Iori's words. "If some of it is set aside to grant your wish, there will be no room to spare!"
"Heh." Iori shook his head, thoroughly unconvinced.
The Einzbern family's wish was "Salvation of all humanity," a grand proposition that required the Third Magic to accomplish.
Originally, Einzbern's Pseudo-Servant of Winter could use the Third Magic to a limited extent. Unfortunately, it was incomplete; the price was steep, its efficiency low, and its practical value limited. It meant little for "all humanity."
That was why they had joined forces with the other two families to create the Greater Holy Grail!
Heroic Spirit summoning and the like were merely means to an end. The Greater Holy Grail's true nature was that of a super Magic Furnace capable of expanding the Third Magic.
However, there was a problem:
If mana were stored purely in a "vessel," then no matter how great that vessel's capacity was, it would eventually reach its limit. It could never meet the demands of