Ciel: "The year was around 300 AD.
This was the era when it's said that Dead Apostles were the most rampant in Europe.
Heretics bearing the curse of immortality: Dead Apostles.
The Church's mission was to turn their bodies back to human, or more accurately, to reduce them to dust by reverting them into corpses.
By reading the words of our Lord's great promulgations to them, sinners feel His love and depart from the earth."
Ciel: "They may look like blades, but they're actually prayers.
Sacraments of the Church, created with the lament for the dead; litanies, as their base.
Thinking about it from a Japanese perspective it's something like, let’s see... a sutra, I guess?"
Ciel: "But then, why do these 'prayers' take the form of blades?
And why are they called 'keys'?
For that, we need to talk about the long storied struggle between Executors and Dead Apostles." 「
Ciel: "Against Dead Apostles, you cannot expect ordinary weapons to have lethal effect.
A wound is something a Dead Apostle can restore through blood;
You cannot kill them by just trying to hack away and tear them apart.
In contrast, Executors are human, even if they can take some amount of punishment. As a result, they are overwhelmed in combat situations by this large difference in survival ability."
Ciel: "Indeed. The only effective strategy against Dead Apostles aren't attacks at a 'single point', but rather attacks at the 'whole surface'; to destroy them with overwhelming destructive force.
But back in that era, there weren't any armaments high-powered enough that they'd be capable of eradicating a Dead Apostle's body in a single strike.
The most effective measure was shutting them into a building or forest, setting it aflame together with everything around it, and then immolating them until their blood had all been used up.
However, if the Church was to do this each and every time, they'd ruin themselves."
Ciel: "So at the time, there was an individual hailed as the pinnacle of both swordsmen and presbyters, who proposed a weapon that was mass-producible, easy to transport, with immediate effectivity while keeping collateral
to a minimum.
These were the Scriptural Weapons, of which Black Keys were the first...
Instead of trying to enchant things with anti-heresy properties by inscribing prayers onto them, they had the groundbreaking idea of instead turning the prayer itself into a weapon, and so render it a potent anti-heresy implement."
Ciel: "For those that have completely abandoned their humanity, of which Dead Apostles are most representative, being exterminated through words isn't possible.
This is because their souls are no longer capable of understanding the meaning of those words.
The ways of man are only applicable to man.
The words of the Lord won’t reach those that have become Dead Apostles."
Ciel: "Accordingly, what we do is not inscribe the prayers onto the soul, but onto the body, thus rewriting its rules into proper ones.
Though the polluted soul cannot be saved, at least the physical body can be turned back to human by these prayers...
Almost as if inserting a key into the empty door of undeath; opening it.
Dead Apostles that are struck by a Scriptural Weapon are compelled to 'remember' their body, even if only that part of them, and then terminate."
Ciel: "And that's the fundamental mechanism of Scriptural Weapons.
Black Keys were created back then, and were the first armament of their kind.
The part that looks like a handle is actually the cut-out page of a manuscript.
Executors use their own magical ene-- excuse me, prayer power, to harden it into the shape of a handle so they can carry it around with them.
During combat, they pour their magica-- excuse me, prayers into it, switching it over into Automatic Baptismal Mode. This 'baptism' piece just coincidentally happens to look like a blade, is all."
Ciel: "During the 9th century, Black Keys settled into the form of today, and were widely used, beloved as the greatest Scriptural Weapon of all.
Many throwing techniques were also developed:
Shadow Pinning, Time Skipping, the Plating Effect, the Cremation Rite, the Internment Rite, Conclave...
However... as far as weapon go, Black Keys are also unreliable, much like the bow and arrow; their effectiveness differs depending on the user's skill and magical energy levels."