The rat catcher's flute is a Gift first introduced in Volume 2 of the light novel during the attack on Salamandra. It is described as a "wind-cutting flute." Later, it is shown to be used by Asuka along with her two other gifts, Deen and Amalthea when Azi Dahaka awakens.
Under Ratten , it was a Gift that controlled the hearts of both humans and beasts alike with its sound.
After Ratten was defeated, it was changed by Jack into a Gift that "by the sound of cutting the wind, told the words of its wielder to others" which would, under someone other than Asuka , be a simple tool of communication. However, under Asuka , coupled with her Gift that gave Divinity to others with her words, the effects were much greater. When coupled with Amalthea and her other Gifts , it is called "Shrine Craft ."
Asuka using the flute in the battle against Azi Dahaka
Ratten playing the flute during the attack on Salamandra