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, and Kiritsugu Emiya had his on his back, while Heine Istari had it on his leg, and Olga Marie's is located on her forehead.
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.
In a sense, Runes, magic jewels, and Command Spells can be considered simplified forms of Magic Crests. The Mage's Association also has the capability of extracting just the Magic Crest from the body and storing it, as was done to Waver Velvet.
Magic Crests are only used by mages who use the style of magecraft used by the Mage's Association. Other magecraft styles and organizations have their own counterparts, such as the Philosophy Keys used by the Spiral Manor in China.
The first generation of a family's Magic Crest is developed by implanting a fragment of some lost Phantasmal Species or Mystic Code, called a 'core,' into the body. Of course, since you are implanting a completely foreign object into the body, the rejection response is far stronger than what one would see when receiving a Crest from a parent. By suffering through those rejections for generations, the core will slowly become saturated with the hosts' magecraft, and thus the Magic Crest would be complete.
However, magi that use this method to produce Magic Crests are all but extinct in modern times.
While there aren't that many from outside previously established families that decide to pursue magecraft, those that do typically received a Crest Split from a much more historied family. Of course, since it's a transplant from a stranger, the intended function of the Magic Crest - to act as a crystallized mystery - is all but lost. Even so, it is a method that produces results many generations faster than starting from scratch, and it is much easier to control the development of a Crest made in such a way.
This method does damage the original parent Crest, but with the help of a Tuner (mages who specialize in healing and improving Crests) such a level of damage can normally be repaired within a few months to a year. In exchange, the source family can expect tremendous loyalty from the recipients, and the family that received the piece of the Crest becomes a branch family of the original. As such, it became common place for families involved in factional struggles to create such branch families, and the original family from which they were derived would come to refer to their Magic Crest as a Source Crest.
There apparently seemed to be some inherent risks and conditions for partitioning a piece of the Magic Crest. As Tokiomi Tohsaka didn't give his daughter Sakura a piece. Whether it was because Tokiomi was a traditionalist, the Tohsaka Crest was to young to be split, his lacking of a trusted tuner, or both his Daughters' inherent talent require a full Magic Crest are unknown.
- For the quests performed by Chaldea, see Grand Order.
Grand Order: Coronal Rank Designation (冠位指定, Kan'i Shitei?), as it relates to Magic Crests, is the highest law of the magus chosen to inherit the Magic Crest, where he is forbidden from committing suicide by a severe mental restraint. Since meaningless suicide of a magus who possesses a Magic Crest would most likely mean his bloodline's loss of claim to magecraft itself, the inability to commit suicide has been known to be the paramount responsibility of magi since the inception of modern magecraft. In order to kill oneself while carrying a Magic Crest, it is necessary to have an extremely legitimate reason.
Although this is what magi know as a matter of fact, the true Grand Order is actually a curse originating from Goetia (under the guise of Solomon) that is crucial for his plan to exterminate mankind. During his lifetime, Goetia planted the curse of Grand Orders in the Magic Crests of select magus bloodlines and designed them so that they would transform the carrier of the Crest into a Demon God after a certain amount of time, completely erasing their original personalities. The prevention of suicide as an ideal was spread to ensure that Goetia's chosen magus bloodlines survived until the designated time. Using this method, he was able to ensure that his loyal servants would appear in certain intervals over 3,000 years to create Singularities for feeding Ars Almadel Salomonis with energy.
In Clock Tower 2015, Lev Lainur Flauros' third personality, Flauros, kills himself in order to prevent his transformation into the final Demon God Flauros, and his remaining two personalities fight over control of the body until they eventually destroy themselves. As a result, Goetia's 3,000-year-old plan to destroy mankind is foiled.
Crest Worms (刻印虫, Kokuinchuu?) are a unique kind of Magic Crest of the Matou family and the personal Familiars from Zouken Matou. Crest Worms are magical energy eating worms that can channel energy even when their host is no longer alive. Anyone implanted with Crest Worms also becomes Zouken’s puppet, physically incapable of defying him. 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. Incase the user is unable to create enough magical energy they will consume their flesh as nutrition or stimulate their desire for intimacy. 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 greatly shortened his lifespan the process in itself was also 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.
 Lord El-Melloi II Case Files material - Encyclopedia: Magic Crest [Magecraft/Other], p.109
Magic Crest [Magecraft/Other]
Passed down through the generations along with a family’s history, the greatest treasure of any magus bloodline.
Like an additional organ added to the body, it won’t be compatible with anyone from outside the magus’ family. In the world of magecraft, the reason older families have such an overwhelming advantage can be narrowed down to primarily because of the number of magic circuits, and the existence of these Magic Crests. The magecraft engraved within that Crest serves both as a magus’ greatest power, and as a curse binding them until they pass it down to their own descendants.
At present, El-Melloi II’s Magic Crest is being held by Melvin. Though some people may have witnessed it for the first time in the last episode of the anime, it is truly nothing more than a “square frame.”
In short, there’s nothing inside of it at all. In exchange for there being no curse tied to it, it contains nothing but what future generations might fill it with. That’s why El-Melloi II is so hell bent on recovering that Magic Crest.
“It’s the number one piece of collateral that guarantees a magus cannot betray you. Taking that away from them is like taking way all the meaning their lives ever had, after all.”
 Fate/side material - Encyclopedia: Thaumaturgy Crest [Magecraft], p.073-074 [T]
Thaumaturgy Crest [Magecraft]
An inheritance passed down within magi families.
The mysteries that a magus has spent his entire life establishing (stabilizing) are turned into a crest and left to his descendants.
It's no exaggeration to say that the crest is engraved with the history of the entire bloodline, and that the magus who inherits it is burdened not only with his family's regrets, but is duty bound to pass it on to a successor as well.
In a way, you could call it a curse passed down through the generations.
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