Why? Alkeides wondered as he fended off the flurry of blows. Why was I wary of this man?
The vortex of magical energy that had sprung up behind him ought to have concerned him more than strikes like these. And yet everything he had accumulated was telling him not to take his eyes off this human.
His strength is certainly more than human, but it hasn’t reached the level of a warrior Heroic Spirit.
So why, he wondered as he continued to take blow after blow. His attention was first drawn to the unnaturalness of his opponent’s attacks.
...Why doesn’t he use his right hand?
Throughout his flurry of unarmed strikes, the police officer called John never attacked with his right hand.
This difference in his center of gravity... A prosthetic?
While attacking and defending at split-second intervals, Alkeides instantaneously deduced the truth behind his opponent’s unnatural movements.
That being the case, he wondered what that prosthetic hand could be.
Does it conceal a weapon? If so, it won’t penetrate this pelt.
No, this man should already know that. Should I assume that it conceals magecraft, then?
Alkeides focused every nerve in his body on John’s right arm while evading his close-quarters attacks.
It must be some kind of—No, is it...?
He felt a presence. A slight presence—an unique magical energy, or perhaps a curse—was escaping from the man’s prosthetic hand. The instant that presence, which retained faint vestiges of the Age of the Gods, tickled Alkeides’ nostrils and skin...
A chill of fear ran down Alkeides’ spine.
Having noticed it, his instincts as a Heroic Spirit made him freeze in shock for just an instant.
No matter how much his Spirit Origin changed, it had a special meaning for him. It was because he knew its dangers better than anyone, because he knew the terror of it better than anyone, that he himself had soaked his special arrowheads in it.
“Damn you...!”
The instant Alkeides shouted, John’s right arm shone darkly...and a peculiarly shaped blade appeared from it, forming the back of his hand.
A black liquid writhed around the prosthetic blade like a curse with a will of its own. It was one of the greatest calamities and most awful curses of the Age of the Gods, one that had killed countless heroes and even driven a certain great hero to take his own life—Hydra venom.

Fate/Strange Fake: Volume 5, Chapter 15 ("Gold & Lions II")