Despite the name, the spell is actually cast by three people - One, the 'innocent child' to be protected, and the other two comprises of a pair of twins that have great synergy with each other.
The 'innocent child' places a piece of unused chalk on the floor and sets up a mirrored ritual, the "horizon" that the twins are separate from. The ritual is done in a way such that the chalk points at the held hands between the twins - Whenever one twin crosses the border set by the chalk, they transform immediately into their weapon counterpart. To finish the ritual, a blue drop of paint is dipped into the chalk to complete the ceremony.
Once the ritual is complete, the spell becomes extremely dangerous - It can be considered one of the most powerful anti-personnel magics in existence.
The 'innocent child' can manipulate which twin transforms into which form - Say a boy holding his twin sword brandishes at the opponent, and is about to receive a severe blow to his gut. By adjusting the chalk's position in a specific manner, the boy can be saved by running the divide, the 'blue ink' area over his general direction. This causes him to instantly vanish and switch with his twin, who holds an axe.
The 'switching' part is no big deal - But the mirrored effect is. Depending on the distance between the twins and the boundary the chalk has established, they won't just 'transform' - They will be 'mirrored'. If the boy with the sword was on the 'left' of the horizon, phasing him out will switch in the girl with her twin axe on the 'right' of the horizon.
Used defensively, it allows for dangerous feint attacks that can lead to counter-plays, forcing the opponent to fight defensively, while boasting a great ability to defend the general vicinity around the 'innocent child'. Used offensively, an unrelenting onslaught of assaults that gives the opponent no breathing space at all is entirely possible.
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Variations of the spell has existed, obviously because of image power. Those of noble status may use their own blood instead of a blue drop of ink, because those of nobility are considered 'blue blood'. This leads to interesting uses of the ritual, too.
If, say, the 'innocent child' obtains an ability to control the drop of blood on the chalk, they can make for interesting twin human or twin weapon plays that may prove very powerful when executed well. This is because the blood represents the "boundary" the twins follow, and by fragmenting the boundary one can make them both humans, or both weapons at the same time.
People known to have used this spell:
Gerrut Tiller (Used with the Pendletons)
Lita Bloodcrust (Used with Marter twins)

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