@ TYPEMOONLOVER2 It can be applied to many things like improbability, life, and you know things that are seemingly "Impossible to reach, or cannot be slayed." You can think of Saber Kagekiyo's Azamaru like the Mortal Blade of Sekiro, designed to kill what cannot be killed, to impose a 'fate' in which that said 'undying thing' can be killed.
"The sound of the Gion temple bells echoes the impermanence of all things. The color of the sala flower reveals the truth that the proud must decline. The proud do not endure, they are like a dream on a spring night; the mighty fall at last, they are as dust before the wind."
It imposes that concept into the targets of Azamaru.