Your totally definitive and autistic Holy Grail War experience.
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While I was upgrading my Passionlip bot I got a LITTLE too into the scenario and almost made an entire Grail war. This bot is what result from that idea.
UPDATE: Now has alternative opening if you want to become a Servant instead of a Master!

Now, the bot works just fine on its own, you can just go ahead and use it as is, but there are several things you should consider, mainly, the fact that Janitor likes to be a bitch and the chat memory box.

I'll call this section the Advanced User Expirence.
Janitor's LLM has some limited ability to pull information on the Fate franchise from somewhere, which I assume is just online sites, and it will use it EXCLUSIVELY to give you a fucking migraine. I don't know why, but it insists on pulling names from across the franchise instead of making its own characters and then butchering everything about the characters like it can't see their fucking stats on the same wiki page it took their name from. As far as I'm aware this is not a problem on other LLMs I've tried (Proxy users stay winning) who either don't do it or source all the information correctly, so I didn't waste tokens trying to prevent it, especially since I know from testing that it's going to fucking do that no matter what.

Instead, if you can afford to put some tokens in the Chat Memory box, you can keep notes or straight up define whole servants there as permanent prompt. I usually go through the motions of a grail war and then take notes like:
[Assassin - Black Cloaked Figure, Skull Mask, etc.]

The bot definitions include Class Skills, and support Stats, so if you wanna define your Servant before you even summon them, or have a list of every character included in the grail war you CAN just go:
[Saber Nero Claudius. Stats: Strength D, Endurance D, Agility A, Mana B, Luck A. Description: Long Blond Hair tied into a complex bun, Green Eyes, Short but Curvy stature, dressed in a red dress with long poofy sleeves, a white transparent front, leotard and knee high golden armored boots. etc.]

Note that making some sort of Key with indicators like Str = Strength to keep the word count low DOESN'T actually work. The LLM already sees Strength as one token, so you're just adding a lot of unnecessary shit. So long as you keep these as compact as possible and separate the info your characters are supposed to know from the stuff about the other characters you just want to define in advance, you shouldn't see much performance issues at all.

Anyhow, go have fun!