"Retaliator, Answerer, all means about the same thing. What Fragarach does is strike first against an enemy's strongest attack...but it has to be invoked after that attack's begun."

'That Which Comes Later Cuts First'. It was the ultimate counter, a Noble Phantasm that all but killed Noble Phantasms. Cu Chulainn used it himself once or twice in his original life, as he recalled.

And somehow, through a couple millennia it had been passed to the hands of a human girl.

"It changes destiny, canceling the attack by killing the enemy before their strike was ever made."

Sighing, he idly leaned his head on his hand once more.

"Now that I'm assuming you get the idea of how that sword works, you want to know why that's important."