Line of Death

The constantly shifting, graffiti-like lines visible to Ryougi Shiki.
The lines appeared on everything, and by slashing the lines with a knife, the object bearing the lines could be “killed.” Since the lines posed no resistance, everything could be killed with the same ease.
The Lines of Death are not the lines at which “an object can be easily cut apart,” rather, they are the concept of longevity given form.
Strictly speaking, it is not “tracing the lines causing the object to fall apart,” but “ending the lifespan and killing the object.”
To put it into simpler terms, it is the erasure of existence rather than physical destruction.
Ryougi Shiki is a living being, and she seemed to have an easier time seeing the end of a living being. This is because as a living being, it is easier to comprehend “the death of a living being.” In order to see the Lines of Death of minerals or Concepts, she either has to become minerals, or use her brain and tap into her “imagination.”
In summary, the end (lines) of an existence that humans cannot comprehend is not visible.

Garden of sinners Pamphlet: Kara no Kyoukai Settings Glossary