There are plenty of magic systems in anime, but few are as densely constructed as that of the Fate Series, where there's a pretty big difference between Magecraft and so-called "True Magic." One small part of Magecraft - called an Origin - shows up now and again in Fate and its affiliated works, serving less often as a tool in combat, and more like a signifier of a character's nature.
Fans of Fate/Zero will remember that in order to get the upper hand against the Mage Kayneth El Melloi Archibald, Kiritsugu fired "Origin Rounds," formed from his own ground rib bones. But it wasn't simply the act of using his own bones to create magic bullets that made them powerful; it was because of the kind of person that Kiritsugu was that his bullets crippled Kayneth as a Master.
Understanding Origin
Put simply, Origin refers to the starting point of one's existence, typically encapsulated in a single word or a phrase that describes the direction one might take throughout their life. It is tied to the "Root," another concept we've covered here on Game ZXC. To make a long story short, the Root is a metaphysical plane where all things past, present, and future in the universe are recorded.
Therefore, an Origin can be thought of like this: if a person were deconstructed into the concepts that define them as a person, their Root is a driving force that stems from the beginning of existence. As such, every human in the Fate Series acts in accordance with their own Origin, though not every character's is disclosed.
At its most basic level, an Origin describes a person's instinctive inclination toward a particular path, rather than a predestined determination of their conscious decision-making. It doesn't suggest the absence of free will. However, if someone's Origin is "awakened," as is seen in the Garden of Sinners series, it's a slightly different story.
When people are awakened to their Origin, it's like becoming bound to their soul's most fundamental desires, turning an instinct into an obsession. In Garden of Sinners, the villain Souren Araya awakens Lio Shirazumi's Origin, "Consumption", which leads him to go mad, desiring to devour the protagonist Shiki Ryougi.
How Origin is Used
As mentioned in the beginning, many Fate fans are likely familiar with Origin through Kiritsugu using it as a weapon. When audiences witness the Origin Rounds being made, Kiritsugu's mentor Natalia describes his origin, "Severing and Binding," using a metaphor about how a thread that has been cut and re-tied will always be thicker where it was cut.
The bullets work similarly. By firing a round at Kayneth's magic barrier, Kiritsugu caused his opponent's Magic Circuits to go completely haywire, damaged beyond repair. Kiritsugu destroyed his opponent's capacity for wielding magic like he was taking scissors to yarn. The threads can be re-tied, but it won't ever be the same as before, like a bone not properly set after being broken.
It's the unique nature of Kiritsugu's Origin that makes it work the way it does, hence why no application of Origin is quite the same in these stories. Oftentimes, it's not something that plays a role in combat so much as it helps understand the core of a character and why they're able to do what they do. Within the lore, even the characters use it to understand others.
There are a few ways in which mages are classified, typically based on their alignment to a particular Affinity, hence why so many mages specialize in particular fields. Lacking a specified Affinity, following one's Origin can yield great results and circumvent barriers that would otherwise hold a mage back.
Fate/Stay Night's protagonist, Shirou Emiya, accomplishes some pretty impressive feats despite lacking a strong bloodline or formal training as a mage. His powers are somewhat basic, but because of his Origin, the amusingly apt "Sword," he is able to stand a chance against opponents like Gilgamesh and even manifest the ability "Unlimited Blade Works."
Most of the characters with confirmed Origins are in the Garden of Sinners series, like Shiki Ryougi ("Emptiness"), Fujino Asagami ("Nothingness"), or Azaka Kokutou ("Taboo"). Mikiya Kokutou, the other protagonist of the story, doesn't have a specified Origin, but it's described as "one that will never hurt others, more so than anyone else."
As for Fate/Stay Night, Rin Tohsaka's is also unspecified but relates to Elemental Affinity, and fans have speculated that it refers to Rin's seeming lack of a specific Affinity. Sakura Matou, on the other hand, possesses the Origin "Imaginary Numbers," which - to put it simply - relates to her powers in the Heaven's Feel route, allowing her to create an imaginary number space to store people and things.
When creating characters or analyzing them after the fact, there's a tendency to strip them down to their base components to figure out what defines them at their core. It's interesting then, that Kinoko Nasu's Fate Series and beyond use Origin as a way to not simplify the characters, but turn a core characteristic into a weapon or a tool that is to be wielded within the story.