(X) Confront them about what they made you swallow.
"Oh?"
Caster puts down their tumbler and looks at you, elbow on table, head tilted on their hand.
"You noticed? Of course, for one who can call upon this Invincible East must not be boring or mundane. It looks like your magical energy is not bad. Actually, for someone your age in this era, it's very impressive. I was expecting you to be at death's door but you've slept for less than a day."
"You can stop with the flattery." There's a definite part of you that doesn't mind the flattery, even if what they're lauding is sort of a tricksy workaround. There's another part of you as well that is always appalled at the alternately lame and grandiose words that come out of your mouth in a crisis but it's not the part that does the fast talking. Either way, this isn't the time. "Answer my question."
Caster's eyes narrow dangerously. Your instinct is to back away, maybe get on your knees and cower or be knocked over backwards and cower, but you refuse. Actually, you hadn't let go of the doorknob yet and that helps. Also, whenever someone tries to get overbearing with you, you've learned a trick where you just pick an eye and stare into it, ignoring everything else. It works. Dongfong Bubai is terrifying but their right pupil alone isn't that scary.
They give a chuckle.
"What you swallowed you already know. You're clever enough to put together the contraption in that other room and to arrive well prepared at this battlefield. You must know about me, who you have summoned and what I am capable of. What you swallowed was the same poison pill I used to control my subordinates after I overthrew the original master and took over the cult. Left unattended and you will die a slow and agonizing death. I have the antidote right here of course." They hold a small black lump up between their fingers. "You can either try to take it. Or, I suppose, you could use a command spell. Of course, if you tried to do it right now, I hope your tongue is quicker than it was yesterday."
Geh.
"What's the point?" you blurt, turning indignation into a spurt of courage.
"The point?"
They tilt their head quizzically.
"There's no point to this," you say, quickly cobbling together words while wondering how you can talk without looking like you're about to use a command spell. The doorknob rattles under your grasp. You're quite certain if you did or if they thought you did, you would lose your left hand in an instant. "We're on the same side."
"And so?"
"So we should be working with each other. Also I'm your anchor in this world."
"Hah. You shouldn't sell yourself so short. An anchor doesn't need arms or legs, nor, come to think of it, a tongue."
"I would bite off my own tongue off before you could and send you back to the Throne with nothing!"
Actually, you would do nothing of the sort and hope fervently it doesn't come to that. But your words seem to have tickled them as they laugh wildly. If you'd awoken now, you'd think you'd summoned a Berserker by mistake. You didn't, but you realize that doesn't mean a given servant of another class is always completely sane. Not that you think Caster is insane, exactly. In life they had both a rational, meticulous, manipulative and ruthless side that brought and kept them in power but also a passionate and emotional side that led to their downfall.
"Then you'd be dead. It's up to you."
"And you'd lose your chance at a wish same as me."
"Why need I have a wish?"
"You have a wish," you affirm. "In order for a Heroic Spirit to answer the call of the Grail, they must have some desire strong enough to be carved into their soul. You might be trying to pretend to be aloof but you aren't. Your existence here proves otherwise."
"You're naive," they say, grasping you under the chin and lifting you up to your tip toes. Their glaring face is right in yours. You didn't even see them stand up or move it was so fast. You reach up and grab their wrist with both fingerless gloved hands. Unsurprisingly it's like grabbing an iron pole.
"Of course I lived with desires and died with desires and so my spirit responds to the call of the Grail. However, right now, I am not merely a soul but a person manifest in this cold, dark world. People are more than desires. Eventually you will understand that some desires are never meant to be fulfilled. They cannot be granted. Cannot be wished for. Should not be wished. I have many unfulfilled desires, but that is not the same as a wish for the Grail."
They drop you to the floor, gasping, fuzzy vision having to refocus as they turn away. They're different now, presenting as a man, though their face is the same, hair bound behind a hat, garish robes of four or five colours over trousers and shoes.
"On the other hand, you and I both know what you want. Power of course. Convinced you are deserving. Convinced that with you it belongs. I have been there. We are all the same. And so, you shall have it. No matter what other heroes are summoned, none can oppose me. Rejoice, girl. Be satisfied with that. Be a good cat and stay here. Do not lift a finger to oppose me or get in my way. When this is over, I'll naturally give you the antidote and you can be on your way."
"I won't let you belittle what I want or how I'm going about it."
You sit back up and raise your left fist. They pause, though they don't turn back to look.
"This is a partnership. I won't allow you to turn down your share. If you don't have a wish, you'd better hurry and think of one."
"Hmph."
They disappear.
You give it a few seconds, breath held, before slumping over backwards onto the floor gasping. You lie there for a minute, wondering what other Servants have been summoned into this Grail War and whether any are as hard to deal with as Caster.
I doubt it, but if I'm wrong, my condolences.
Still, you didn't get to where you are now by lamenting mistakes. You get up, hit the bedroom light switch and move the laptop to the little desk where you plug it in. In fact, this isn't even a mistake. You knew from the start that they'd be hard to deal with. However, you also wanted strength. So here you are now. It's time to take stock.
I want to follow Caster out but it's not worth doing just to upset them. I've been in suspended animation for most of the time the other participants will have been chosen and begun preparing. I'm way behind on intelligence and logistics. Master-Servant relations will have to wait.
[ ] Search online for news and rumours and so forth and research any leads you find.
[ ] Catch up on your mail and communications, talk to your connections and make inquiries. Riskier, especially if someone catches on and spills that you're here, but you're pretty confident in your tech abilities.
[ ] Deploy the drones. There's no online substitute for active reconnaissance and you might catch action as it happens.