Engraved on the body like 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 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 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 stored inside of it, 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 prana 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 prana 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 prana 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 errupt inside 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, prana 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 prana 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 - Magus: Magic crest, p.041
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.
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