There are no Rulers in this Great Grail War. One Ruler has not been summoned because this war will not affect the world, and another Ruler is already dead. There will be no new Rulers called.
This world is just a remnant, repeating over and over until a miracle is recreated and glory is achieved.
You have been called by Fafnir because you are the Master who restored the Human Order. Fafnir used the power of the Holy Grail to summon you in the form of a dream. Right now you are akin to a spiritual body. Your original self is still travelling in the Shadow Border.
You ask Fafnir for an explanation and he says it's a bit hard to explain. Fafnir: "…it's a bit too complicated… wait a bit, let me organize my thoughts, I'm gonna sort out an explanation…"
First he explains how you're currently in the Far Side of the world, where phantasmal species have come to rest. The Human Order envelops the planet like a cloth and establishes the world humanity lives in. If the Human Order is disrupted, phantasmal species may appear there. Guda: "So it's like… in the middle of a conversation, suddenly, wyverns." Fafnir: "Yup, yup, that's right."
Currently this place is in crisis because of the Greater Grail that Fafnir brought. It's damaged from a certain NP but Fafnir says it was fine until one day it suddenly broke. Fafnir: "Somehow, even though I didn't do anything, it broke." Guda: "That's what the culprit usually says."
Inside the Greater Grail, Holy Grail Wars have started up. Over 10,000 wars have been waged. The first battle had 2 Servants duking it out, the 567th had 4, and in the 2330th repetition they got to 7. Fafnir's control over the Greater Grail is also slipping. He thinks someone is interfering in order to get their hands on the Grail.Post too long. Click here to view the full text.