Magic Crest

Thaumaturgical Crests (魔術刻印 - Majutsu Kokuin) are the most important treasure of a lineage of magi. Engraved on the body like a magic circle, Magic Circuits that were given a more stable form in order to act as an archive of thaumaturgical knowledge. 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 is the number of Circuits forming the Crest and greater is 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: Tohsaka Rin has hers on her left arm, while Emiya Kiritsugu had his on his back.

Although being a independent set of Magic Circuits than those natural of a magus’ body that can’t even be detected when there is no prana being passed through them, transplantation of a Thaumaturgical 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 Thaumaturgical Crest is still a highly valued article for any magus. But since usually only one can inherit, troubles can arise when there is more than one candidate. It’s not unheard of bloody feuds happening inside a clan of magi because of a dispute of 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 and magic jewels can be considered simplified forms of Thaumaturgical Crests.

Crest Worms
Crest Worms (刻印虫 - Kokuinchuu) are an unique kind of Thaumaturgical Crest of the Matou family made out from Matou Zouken 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.

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 Matou Kariya. However, doing so not only will greatly shorten his remaining lifespan but also the process is in itself incredibly painful.

The worms implanted in Matou Sakura are special for they have the remains 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.