Gregorian Chant

The Gregorian Chant or Gregorio's Chant (グレゴリオの聖歌隊) originally refers to the grand spell used by the Roman Catholic Church, which uses 3,333 monks chanting simultaneously to conjure a powerful long-range spell similar to that of an orbital bormbardment.

However, it is more accurate to describe it as a simultaneous chanting of a single spell by a mass of people.

Etymology
The term is originally used for a form of liturgical music in Western Christianity Masses and rituals as primary accompaniment. It is named after, Bishop of Rome from 590 to 604, who is traditionally credited for having ordered the simplification and cataloging of music assigned to specific celebrations in the church calendar.

Original Gregorian Chant
The Original use of the Gregorio Chant was a weapon used by the Roman Catholic Church, using 3,333 monks within a Vatican cathedral, requiring a core for the monks to chant simultaneously. It is apparent that the more members of the choirs have, the more powerful the spell would become. 3,333 monks alone is powerful enough to result in a bombardment spell that can raze a multi-story building.

The Thirteen Knights of the Roman Catholic Church's role with the Gregorian Chant is seemingly to have the members triangulate and pinpoint the location where the bombardment should take place, in this way, the monks are the conjuring and chanting the spell, while the knights are merely the spellcaster. The spell can be described as a powerful pillar of red light similar to lightning tearing the sky asunder and striking the target.

Chants
It is unknown if the chant required for the Gregorian Replica to work is the same, however, the knights require someone from their ranks (most likely their leader) to recite a verse from the book of Revelations of the New Testmanent to cast the spell with their sword raised. According to Touma, the sword seemingly act as an antenna and glows red upon raising for the spell.

"Revelation 8:7, the first angel sounded his trumpet, and there came hail and fire mixed with blood, and it was hurled down onto the earth!"

In the original light novel, Vittorio Cassera was the one who recited the verse as the knight Lancelot. In the anime adaptation, an unnamed knight (presumably filling in for the character Vittorio) recites the verse. Immediately, after a sound of a horn can be heard from the sword, apparently due to the effects of the magic; the exact reasons however, are still unknown.

Gregorian Replica
The Gregorian Replica (偽・聖歌隊) was Aureolus Izzard's attempts at replicating the original. Here, instead of 3,333 monks chanting simultaneously, Aureolus instead uses the 2,000 brainwashed students of Misawa Cram School as the chanters.

Like the original, Aureolus is forced to use a core to control them, however, this where the similarities end. Apart from the fact that they are not monks and are aware of magic, the students are espers which seriously injure them whenever they are forced to use the spell. Moreover, due to these limitations, the form of the spell significantly looks different to the one used by the Roman Catholic Church wields. Instead of a red pillar of light that strikes the target, the form the Gregorian Replica takes is that of a small bluish ball that floats and follows the target, each ball corresponds to one of the students.

Despite it's small size, the sheer number of the bluish balls and the fact that it is still a replica of a powerful spell, Stiyl claims that it should not be underestimated.

Chants
It is unknown whether the chants used by the espers are the same one that are used by the monks for the Original Gregorian Chant. The chant introduces a subject at the end of each independent clause where it is then used as the subject for the next one until the end where the first subject is reused for the final one. That kind of rhyme is called a chain rhyme.

"The wings of the seraph shines brightly."

"The bright light is a pure white color that reveals all sins."

"The pure white color is the mark of purity."

"The mark is the result of one's actions."

"The result is the future."

"The future is time."

"Time is uniform."

"Uniformity is all."

"All is created by the past."

"The past is the cause."

"Cause is one."

"One is sin."

"Sins are humans."

"Humans fear, and fear is guilt."

"Guilt resides within oneself."

"Oneself if there is something one despises within, the wings of the seraph shall reveal your sins, and remove it from within you."

A solo chant is performed by an esper girl Touma meets in Misawa Cram School. It follows a similar grammatical structure to the one above, and is possibly a continuation.

"The flames punish sin." "Purgatory governs the flames." "Purgatory was created to burn sinners, as the only violence God recognises."

"Violence is the affirmation of death." "Affirmation is recognition." "Recognition rests within." "Within refers to the world." "Connect your inner self with the outside world."

Ars Magna
Aureolus manage to achieve Ars Magna through the use of the Gregorian Replica. Normally, the ritual required to theoretically achieve Ars Magna would take centuries to achieve, needing descendants or at least for the performers of the ritual to be immortal to finish it.

However, by using the 2,000 people of the Misawa Cram School to make them chant at the same time, the speed of chanting will be at 2,000 times. With this, Aureolus managed to achieve the pinnacle of Alchemy and increasing its effects 120 times at the cost of killing the chanters, which Aureolus manages to bypass by simply reviving them. It should be noted that for this case the casters where Espers that by definition can't use magic; and because of that its unknown if regular people or magicians would be able to perform the ritual chant for Ars Magna, following that exact method and without dying.

Deep Blood Arc
The spell is prominently featured in the arc, with a replica being used by Aureolus Izzard to attack both Touma and Stiyl during their infiltration of Misawa Cram School. Here, instead of 3,333 monks, Aureolus Izzard instead uses the 2,000 students for the spell, and as a result seriously injure them since they're espers.

Later, the true Gregorian Chant is used by the Thirteen Knights of the Roman Catholic Church, to bombard the entire building, however, it was in vain, as Aureolus simply used his Ars Magna to reverse its affects. With the spell's defeat by Aureolus, the Church would later go on to have a power struggle.

It is later revealed that Aureolus used the Gregorian Chant done by the students of Misawa Cram School to enable to him to use Ars Magna, which originally required for the chant to be passed down from chanter to chanter to be finished, since the spell would take hundreds of years to finish chanting. Here, Aureolus simply revives the students who die when using magic with his Ars Magna, this way the students can continue chanting.