It is the source of all supernatural phenomena such as Magecraft or ESP, and beings such as True Ancestors, Dead Apostles, Servants, Spiritual Bodies, as well as Phantasmal Species show signs of Mystery. Mystery accumulates over time and grows stronger with age.
If two different types of Mysteries collide with one another, the stronger Mystery will overwhelm the weaker one. It is through this law that Lugh Beowulf is able to remain invincible against modern magecraft; because he has 3,000 years' worth of Mystery, which is far more than any modern magecraft has, he is able to simply ignore any effects that magecraft would have on him.
Certain items that have a long history and age can also show signs of Mystery, as in the case of the centuries-old katana that the Ryougi possess, Kanesada Kuji. If they are severely damaged, however, they will lose all the Mystery they have accumulated.
It is also possible for a human's body to reach the level of a Mystery, such as with Shiki Ryougi. There are also the cases, of Doctor Heartless a magus who got his heart stolen by Fairies and the Mixed-Blood Kouma Kishima a true Crimson Red Vermillion, who have both become a mystery.
 Fate/complete material III: World material - FAQ with Nasu: Miscellaneous, p.136
Q: Who would be stronger if Servants fought the 27 Ancestors?
Q: Who would be stronger if Servants fought the 27 Ancestors? Servant's also have superhuman battle ability, but the 27 Ancestors are also superhuman monsters.... I got the impression that ORT, Primate Murder, Altrouge Brunestud, and so on were obviously stronger than Servants.
A: All I can is they're both powerful.... They're both beings that live in mystery, so the only thing that can be said is that it all depends on the situation the moment it comes done to ""having the means of killing the other guy somehow". Among those, Earth rules won't even apply to ORT in the first place and Primate Murder has the super advantage against primates so they're in a league of their own, I suppose. Speaking of which, for one Primate Murder, seven Guardians would be the appropriate amount.
 Kara no Kyoukai Special Pamphlet - Encyclopedia: Magecraft [Others], p.038-039 [T]
A general term representing activities that artificially re-enact Mysteries/Miracles.
Although there were some differences depending on traditions and schools, the basic principle was still "the transformation of the magical energy that exists within the practitioner or the environment".
The practitioner would issue Commands according to the Foundation (System) of his school, and this would cause the execution of a predetermined Program.
Magical energy was the electrical current required to deliver the command.
Although magecraft gave the impression of omnipotence, it fundamentally produced Mystery through the principle of equivalent exchange.
It was possible to transform one thing into another, but it was impossible to produce something from nothing.
However, the essence of magecraft as a discipline was to challenge that "Nothingness", and endeavor towards the impossible. Large-scale spells such as the so-called Grand Sorceries, Grand Rituals were really purposeless if not for attempting to reach " " (Kara) or True Magic.
Kara no Kyoukai was a story of one magus' attempt at challenging "Nothingness".
- ↑ Fate/stay night Fate Route - It is the most fundamental rule that a weaker Mystery is erased by a stronger one.
 Kara no Kyoukai - Chapter 5: “Paradox Spiral” Movies Q & A Pamphlets
Q: Will it be possible to repair the antique katana now that it’s broken?
Q: Will it be possible to repair the antique katana now that it’s broken? <Yasunosuke>
Nasu: Even if it was reforged, the years it had accumulated before would be lost.
Takeuchi: A lot of people asked this question. Since I thought it was going to be Shiki’s final weapon, too, I was pretty shocked when it broke almost as soon as it finally appeared!
Nasu: That’s how ultimate weapons should be.
- ↑ Melty Blood - A composition that is neither natural nor artificial. Eastern ideology can be profound at times. It can create people like you who has attained the level of being a Mystery while still being human.