Suddenly, intense pain burned at the base of Kiritsugu’s little finger; his back stiffened.

“…?!”

When he had truly started treating Hisau Maiya as his assistant, Kiritsugu had put a spell on a strand of her hair and embedded it in the subcutaneous tissue of his little finger. At the same time, Maiya had also embedded a strand of Kiritsugu’s hair in her finger. If the Magic Circuits of one of them entered a state of extreme stagnation—the stage at which one's life force was weak on the verge of impasse with death—the strand of hair given to the other person would burn, warning the other and notifying them of the existence of danger.

That was a measure implemented for a worst-case scenario in which it was already impossible to use a wireless network or familiar to convey information. That is to say, it was only a signal to inform that "it is already too late". For it to activate now, at this moment, what exactly could it mean…?

Before wavering, one must first be caught in a quandary. Kiritsugu mobilized all his neurons and began to ponder the situation at hand as well as to formulate response plans.

Maiya was on the verge of death—in other words, this meant that Irisviel, hidden in the underground storage, was in danger. The sequence of events and its cause could not be determined now.

- Fate/Zero: Act 14, Part 1