World-building can make or break a franchise, especially those reliant on the escapism found in fantastical societies, and the Fate franchise has a lore that feels significant beyond simply "Fate." The building blocks that made that series popular are but smaller pieces of an intricate world, and at the center is one of the most important aspects of all.
The Holy Grail War in Fate/Stay Night is understood to be a battle wherein seven Masters each summon a Heroic spirit to fight to the death until one master remains and gets their wish granted. However, the Holy Grail War is but one messy way that characters in the Nasuverse have attempted to reach a power known as "The Swirl of the Root," or the Root for short.
The Akashic Records
It has gone by many names depending on the story and translation, but the name that has been the most effective at describing its power is the Akashic Records, in reference to anthroposophy. The Records are a theoretical record of all events, thoughts, feelings, words, and intent across time, past, present, and future. Characters that have attempted to reach it see a connection to the Root as attaining a sort of godhood.
In Nasuverse lore, Magic isn't the same as Magecraft, as funny as that might sound. Magecraft implies performing acts that could technically be accomplished through physical means, only faster and without the material process. A magus creating a fireball is a wonder to one who has never seen magic, but it's only a recreation of a preexisting phenomenon.
But "Magic" in the world of Fate implies something that is impossible to replicate no matter the time or the materials in a given environment. Magi can use Magecraft, but true Magic requires getting past the metaphorical wall separating people from impossibility. Characters in Fate occasionally discuss things like the "First Magic", "Second Magic", and so on. These are documented examples of Magic from beyond this "wall" that remain highly vague and difficult to comprehend.
The Path To The Root
Reaching the root, or "Akasha," is the primary goal of the Magi, and their research is often heavily protected and secretive because of that. Some Magic exists because of previous magi who found a path to the root, such as the First Magic and Fifth Magic. Other forms exist as ways to open up the pathway in themselves.
Most magi spend their whole careers researching particular fields of Magecraft, hoping that by reaching the pinnacle of a concept, a path to Akasha might present itself. Alternatively, Magi have manipulated certain systems to open a path, hence where the Holy Grail War comes in. Because Akasha is a record of all life and even stories, the Grail draws from the Root to create Heroic Spirits, so one would conclude that the Grail War presents an opportunity to access the Root.
People who harness Magecraft are Magi, but those whose powers are drawn directly from it are true Magicians in every sense of the word. However, while some have strived to reach Akasha to gain Magic, it isn't the ultimate goal. The objective is to find the truth of the universe and - perhaps most importantly - to understand it.
There have been characters who have found a path to the Root and have become full-fledged Magicians. Aoko Aozaki from Mahoyo and Tsukihime is significant because while she isn't an exceptional Magus, the power she eventually inherits is directly from the Root. But that isn't to say that she has attained a complete understanding of Akasha, and with how secretive the Magi are, she doesn't share much of what she does know.
Ryougi Shiki, the protagonist of Garden of Sinners, is also connected to the Root, which accounts for her power, "Mystic Eyes of Death Perception." This is because Shiki's origin (a word that defines someone's existence and compulsion towards acts) is " " (emptiness), the same word (or lack thereof) used to describe the Root, as its name is unpronounceable.
Touko Aozaki, another Garden of Sinners character and the older sister of Aoko Aozaki from Mahoyo, once strived to reach the Root but gave up before her compatriots. In her pursuit of creating perfect replicas of human bodies, she believed that humans had become too diversified genetically to reach the Root.
In the fifth Garden of Sinners film, Paradox Spiral, Touko's old classmate and rival, Souren Araya, sought to open a path through a grotesque experiment. He used a concept developed by Touko to put innocent people inside a time loop. They would live out a cycle of death, and then Araya would use Shiki's power to create a path to Akasha.
The Importance of the Unknown
Any fan that's looked up lore about the magic in Fate has probably found themselves more than a little bit confused because it's difficult to discern exactly what it even is. Some can be explained somewhat, such as how the Second Magic relates to Parallel Worlds, the Third pertains to the materialization of the soul known as Heaven's Feel, and the Fifth functions like Time Travel.
However, those are merely surface-level signifiers of those Magics, not a thorough explanation. The nature of magic/magecraft in this franchise is intentionally obscure and at times paradoxical because it takes a somewhat academic approach to quantify and qualify the supernatural. These concepts are as unknowable within the text as questions about the afterlife are to audiences.
Normally, audiences can suspend disbelief and generally take certain elements for granted, but the Fate franchise goes out of its way to ground modern fantasy by demystifying it. Even in a world where humans have magic, there is still something to strive towards that remains out of sight and out of reach, which somehow makes the world feel more relatable without losing its magic.
There is an idea within the lore that everything supernatural in the Fate universe only exists because of "Mystery" and humanity's ability to imagine something beyond explanation. If something is known and thus "demystified," it begins to lose power. It's an incredibly profound notion for a work of fiction, and one that Fate tackles subtly, mostly in the background.
It's not an excuse to write poorly or a cop-out, but an honest assessment of why things like magic are so alluring to fans of fantasy escapism. Some things are simply beyond comprehension and though we can observe and attribute rules, there will always be limits. Even as a background element that some fans won't notice, this approach to world-building is foundational to every aspect of this franchise and speaks to the passion behind its creation.