Rin and Shirou are having supper... Shirou tells her his reasoning for his ideals.. she agrees to still be his partner even though she's against continuing to fight. She knows she can't stop him anyway.
They come up with a plan to ally with Illyasviel, Berserker's master... Rin knows the Einzburn mansion's location due to her family ties...
Caster and Archer discuss plans.. they know the priest still has his trump card servant.. suddenly a flashback to Caster's previous master learning to steal mana from humans. She shows her master how to drain mana much more efficiently.
"How can you drain mana from an empty kiln?"
"Empty? Oh... So this appears empty to the eyes of you modern mages? Your mana-generating technique must be one step... No, one dimension removed from mine."
- she has like, 3rd eye/4th dimensional shit going on
"I have no need for incantations. To me, magic is simply something to command."
"Throwing away lives meaninglessly is unwise. As a mage, I cannot abide a workshop that generates such a loss."
her master uses a command spell to keep her from betraying him. then he goes to the priest... Caster is upset that he called her a witch.. just like everybody else..
"After all, this woman killed her romantic rival by burning her alive in her castle."
The master hands Kotomine a letter.. a contract selling the right to dispose of Caster. "If she agrees, I'll be rid of Caster without having to waste a Command Seal."
The master returns to find Caster burning down his workshop.. part of a ritual... she has found the place where the Greater Grail rests... a natural fortress that repels Servants.
he uses up both his command seals to get her to kill herself, but she already used the rule breaker to sever her pact with him.. he wasted his seals for nothing... he called her a witch, before she burned him alive inside a dark maze with weird wall paintings.. then Lancer shows up and... I guess ruins her plans to harvest her dead master's mana?
... and the twist... that was actually Caster's original master in the current grail war... that's why she remembers all this... after that, she was saved by Kuzuki and he formed a pact with her. Kuzuki agrees that she needs sacrifices to sustain herself, because he can't act as a proper master.
Illya wants to let them in... she knows about her family ties with Shirou...
Shinji has also come to talk with Illya, but they don't let him in... then Gil shows up... they recognize him as a king but still don't trust him.
"Shirou is coming here, to the castle where Mother once stayed."
Illya and Berserker show up just after he kills them..
"Shall we reenact the battle of legend here"