"Wait, a damn minute... Where did Voli get the inspiration to create this kind of technique?" By the description of the technique, Fein could determine that the technique was actually powerful. It was a deadly penetrative attack good for enemies with tough defense and also excellent for assassination.
Of course, Fein also felt that the tweeting of the birds was a drawback. After all, in the assassination, soundless attacks are the best. The tweeting of the birds would make the enemy aware and could also reveal the location of the user. 'Hmmmm... Since Voli could cast this ability instantaneously, this drawback would be minimized.'