Minor Spoilers Ahead for Fate/Stay Night, Witch on the Holy Night, Garden of Sinners, and Tsukihime.
Magic in the Fate universe can be a tad confusing at times based sheerly on the expansiveness of magecraft, the different fields within, and how it all connects to the very existential root of its universe. But one particular kind of magic pops up frequently throughout Fate and especially its associated works, and those are Mystic Eyes.
They are the oldest form of Magecraft in the Fate universe and manifest in many forms, possessing different abilities and ranked accordingly. They seem to show up in just about every iteration of the franchise, yet manage to retain an air of significance no matter how often they appear thanks to the status attributed to them, to say nothing of their variety.
Understanding Mystic Eyes
In Fate lore, magic is largely thought to exist because of the existence of superstitions, the fixation on the unknown, and the logos associated with symbols, myths, and legends. Similarly, eyes have been used as symbols throughout history, and even are used to characterize forces of nature, such as the "eye of a storm."
Mystic Eyes are a form of magic that is born from the power attributed to the eye and tend to come about due to mutations in a person's mana circuits around their eye. It's possible to create artificial ones, but those can only be made so powerful compared to the genuine article. And it's not like having them is an enviable position in every instance.
They can come with serious downsides. The powers can be unpredictable, or they might activate by themselves. Even if the effects don't harm others, they can make life difficult for the wielders. When Tohno Shiki and Ryougi Shiki acquire their Mystic Eyes in their respective stories, both of them are unable to control their power.
Ryougi is the protagonist of Garden of Sinners, a prototype of Tsukihime, in which Tohno is the protagonist. As such, they both possess the Mystic Eyes of Death Perception, which lets them see lines of "death" in every object and person around them. By cutting along these lines, they can bifurcate anything and everything, albeit with some differences between the two characters' capabilities with their powers.
When people think of Mystic Eyes, they tend to think of either Garden of Sinners or Tsukihime, largely because these powers are a huge focal point. After all, Death Perception is ranked "Rainbow-class," the highest ranking of all Mystic Eyes, making Ryougi and Shiki that much more significant as protagonists. But different kinds of eyes appear all throughout the Fate universe.
Mystic Eyes Throughout The Fate Universe
For starters, one of Fate/Stay Night's main servants, Rider, possesses the Mystic Eyes of Petrification, a Jewel-Class set which - as the name suggests - turns people to stone and absorbs life force. It's a reflection of Rider's identity as the Heroic Spirit Medusa and no doubt is the reason for the eye's incredible power.
The ranking system of Mystic Eyes is referred to as the Noble Colors. Red or green eyes are normal kinds, while Gold-Class is stronger but typically held by vampires. The most powerful are either Jewel or Rainbow-Class, though Death Perception is the only Rainbow one that exists in the lore. The higher in power, the rarer they get.
Additionally, it might surprise fans to learn just how many characters possess Mystic Eyes. Fans of Garden of Sinners think of Ryougi as the wielder of special eyes, but Touko Aozaki also possesses the Mystic Eye of Enchantment, a common type. This is revealed in Witch on the Holy Night, in which Alice Kuonji also possesses Enchantment, though in both of their cases, only their left eyes, possess this power.
For any fan who is intrigued by these powers and wants to see more of them in action, Lord El Melloi II's Case Files is a must-watch. It's effectively a sequel to Fate/Zero following a grown-up Waver Velvet teaching Modern Magecraft theory and taking on magical-related investigations. But a big crux of the story focuses on the Rail Zeppelin, a train that collects Mystic Eyes.
Case Files culminates in a murder mystery above a magical train that rides through the forests of Europe, where these eyes are auctioned off. As stated before, Mystic Eyes can't be totally manufactured, but aboard the Rail Zeppelin, they can safely be removed from one host and given transplanted to another.
Audiences get further insight into the expansiveness of Mystic Eyes, how highly sought after they are, and get to witness different wielders of them and the unique and clever abilities at their disposal. Few modern fantasies are as dense with their magic systems as Fate, and those who are in love with that complexity will fall in love with Case Files.
Mystic Eyes are just one part of the magic system at play, but even as just one small piece of the grander tapestry, they have versatility and mystique about them. Whether they're the most powerful or just a handy trick for characters to dispense, they never fail to change the playing field and create more interesting stories and action.