Alkeides had no way of knowing exactly what that “black mist” was.
Still, all his accumulated experience and the senses honed by the deadly combat he had just been engaged in told him that it was no ordinary being.
He could not know what became of those engulfed by the inky blackness, but he did notice one thing: The Spirit Origin of Kerberos, a part of his Noble Phantasm, had vanished from the area. The magical energy that linked them was not completely gone, but he could neither recall it nor dismiss it.
It was as if a massive ward was slithering of its own accord in an effort to isolate him.

Fate/Strange Fake: Volume 6, Chapter 16 ("The Canon of the Demigods II")