Rider of Red’s—Achilles’ mother, the goddess Thetis, had tried to extinguish his human blood and turn him completely into a god by bathing him in holy flames when he was a baby. Though it had been stopped midway by her husband Peleus, his entire body except his heel became immortal.

As a result, no form of attack would have any effect on Achilles, but… that special characteristic of his had two pitfalls. First, he could be wounded by those with the blood of the gods like himself. And second, when faced not with an attack, but instead—

“…Against an act that displays friendship, my invincibility does not apply.”

Rider of Red made a displeased expression as he finished Archer’s explanation himself. Archer of Black nodded without smiling.

- Vol 2 Chapter 3