Engraved on the body in a variety of shapes, most typically in a magic circle, a Magic Crest is a series of Magic Circuits that were given a more stable form in order to act as an archive of thaumaturgical capability. At some point of the magus’ life, he will forge some of his own Magic Circuits into the Crest, store many, if not all, spells that he learned in life and then pass it down to his successor. As the process repeats itself with each new generation, the older a lineage is, the greater the number of Circuits forming the Crest, and the greater the amount of knowledge stored inside it. It is the duty of any heir of a family of magi to successfully expand and pass down the Crest to the next generation.
As the complexity of the Crest increases with each new generation, the spells stored inside them become more stable and easier to activate. As long as the Crest is completely integrated with the magus' body, he will be able to cast any spell recorded in the crest, even if he himself never had the chance to learn it properly. All he has to do is to activate the Crest just like he would do to a normal Circuit (by running magical energy inside of it), find the spell stored inside and activate it with its relevant incantation and thaumaturgical process. Furthermore, it is possible to use it to support the magus' regular spellcasting and, whenever the magus gets heavily injured, the Crest is capable of keeping him alive for as long as there is still magical energy inside his body. Location of the Crest varies: Rin Tohsaka has hers on her left arm, while Kiritsugu Emiya had his on his back.
Although the Crest is a set of Magic Circuits independent from those natural to a magus’ body, and cannot be detected when no magical energy is being passed through them, transplantation of a Magic Crest is still very much like that of an internal organ and thus can only be done to a compatible host. Normally that would mean someone who shares blood relation with the magus, but that is not an unbreakable standard. Even between close relatives, the chances of rejection are high. Therefore, in order to increase the chances of success, the transplant is done slowly throughout the heir’s childhood, so that their bodies can get accustomed to the presence of the Crest as they mature. Furthermore, the use of medicines made of special herbs and crushed bone is normal during the whole process, although bloodlines with a long history of 500 years or more have achieved a genetic uniformity that allowed them to overcome the need for that altogether.
Despite all the troubles, the Magic Crest is still a highly valued article for any magus. But since usually only one person can inherit it, troubles can arise when there is more than one candidate. It’s not unheard of for bloody feuds to erupt within a clan of magi because of a dispute between two or more children fighting over the family’s Crest. It was because of that and other reasons that the current trend of only one heir per family (and the consequential policy of forsaking or excluding all other children) came to be.
Crest Worms (刻印虫, Kokuinchuu?) are a unique kind of Magic Crest of the Matou family made out from Zouken Matou's personal Familiars, magical energy eating worms that can channel energy even when their host is no longer alive. So far the only known example of a Crest that can be passed down to someone outside of the bloodline, forcibly confers to one’s body the capacity to use Magecraft.
Crest Worms are not only capable of expanding one’s natural Magic Circuits, but also capable acting as artificial Circuits after a certain incubation period is complete. Since they usually remain in a dormant state, the worms don’t have that much direct influence over someone. However, once awakened by the activation of Circuits they will try to force the body to continuously produce magical energy in order to sustain themselves. Through the use of these and an intensive training, it is possible for someone with no prior experience in Magecraft to become a magus eligible to participate in the Heaven's Feel in roughly a year, as was the case with Kariya Matou. However, doing so not only will greatly shorten his remaining lifespan but also the process is in itself incredibly painful.
The worms implanted in Sakura Matou are unique since they have the fragments of the Holy Grail summoned in the 4th Grail War as component materials. After an incubation period of almost 11 years, the worms have spread themselves through the girl’s entire body.
 Fate/complete material III: World Material - Theory of Magic - Magecraft: Magus, p.041
Magic circuits are the pseudo nerves that a magus has inside himself. These are paths that generate magic energy and access the magic foundation engraved in the world. Magi make magic energy with the magic circuits and use the magic energy to run the magic foundation and thus put magery into effect. To do either of these, the magic circuits are necessary.
As suggested by the description pseudo nerves, these are split into cores and lines joining these together and are spread throughout the body.
The number of magic circuits are fixed at birth and the number won't increase by other than external factors.
The product of stamping the anchored mystery that a magus had spent his entire life building up. Can be said to be mystical inheritance that defines the magus line as a magus line. While the inheritor is young, it is necessary to implant this little at a time while at the same time having the body adjust to the magic crest by medicine and rituals. The magic association also has technology for extracting just the magic crest from the body and storing it.
What determines what kind of attributes a magus' magic is going to be likely to have, or what kind of magic the magus is going to have good chemistry with, is going to be the element aligned with the magus.
In addition to the five great elements of earth, water, fire, wind, and sky, there's the imaginary elements of Hollow and Nothing, for a number of seven.
Some of the five elements will differ according to the school of magic. A famous one is a style in which the five great elements are formed from wood, fire, soil, metal,and water.
Basically, one magus has one alignment, but sometimes there are magi with two alignments (like fire and earth, wind and water) and there are also those called "Average Ones" who have all the five great elements.
Matou Sakura normally would have been a holder of the imaginary element of Emptiness. However, through her magic training, hers had been changed to the water alignment of the Matou line, so she couldn't exhibit her true talent.
Five great elements Earth One of the two alignments of Kiritsugu. Kiritsugu exhibited his attribute of combining and used magic which combined the wounds of an enemy in a different form than normal, and made them unhealable.
Five great elements Water The alignment that the Matou magi have had for generations. Matou exhibit the attribute of absorption and have passed it on. Sakura's alignment changed from Emptiness to water.
Five great elements Fire The alignment of Tokiomi and also one of the two alignments of Kiritsugu. It is the symbol of life and death and has good chemistry with destructive magic.
Five great elements Wind One of the two alignments of Kayneth Elmelloi Archibold. There are few magi with wind as an alignment
Five great elements Sky An element which makes up celestial bodies that is newly added as a fifth element to the four great elements of earth, water, fire, and wind. This is, in other words, Ether.
Imaginary Elements Hollow The alignment that Sakura was born with. Defined, in magery, as that "which is possible but is not in the physical plane". Also called imaginary numbers.
Imaginary Elements Nothingness Like Hollow, the alignment of an imaginary element. In magery, defined as that "which is not possible, but materializes". Has a different meaning that the "nothing" of physics and math.
•The absoluteness called Origin
If alignment determines the root of one's magic, then even deeper than that, is the origin which determined the root of one's existence. The origin is not only for magi, but is the orientation that all things are born with and it's existence itself is thought absolutely necessary.
Those magi with an origin that is strongly expressed outwardly are sometimes removed from the normal alignments and there are times that the origin itself becomes the alignment. Most of those magi exhibit their talents as extreme specialists.
Because of that, it's said that they can reach higher grounds than normal magi.
Of the alignments, wind is called "noble" and fire is "normal". By the way, the term "Noble Colors" is used to refer to special and innate things where he workings of the magic circuit are close to the movements of a celestial body (normally they're closer to the movements of the crust), and is innate and special.
Noble Colors are often used to refer to mystic eyes, and Rider's petrifying mystic eyes are also a noble color.
|Bounded Field • Conceptual Weapon • Gradation Air • Magic • (Second Magic • Third Magic • Fifth Magic) • Magical Spells • Marble Phantasm • Mystic Code • Mystic Eyes • Mystic Eyes of Death Perception • Psychic • Reality Marble|
|Demons • Divine Spirit • Dragon • Familiar (Dragon Tooth Warriors) • Homunculus • Humans • Magician • Magus • Phantasmal Species • Spirits • Vampire|
|Holy Grail War|
|Command Spells • Great Grail • Heroic Spirit • Holy Grail • Master • Noble Phantasm • Parameter Rules • Servant • Servant classes • Skill|
|Church • Demon Hunter Organization • Mage's Association • The Twenty-seven Dead Apostle Ancestors|
|Akasha • Chants and Incantations • Elements • World • Magic Circuit • Magical Energy • Soul • Magic Crest|