the Face In The Floor: the Substance unmasked — "has finally removed its smily-face mask…"
has finally removed its smily-face mask to reveal centerless eyes and a ravening maw, and canines down to here, it’s the Face In The Floor, the grinning root-white face of your worst nightmares, and the face is your own face in the mirror, now, it’s you, the Substance has devoured or replaced and become you
Context
The addiction-as-horror-story climax, and the theme's central identity-claim. The Substance is figured throughout as a beloved ("your old friend," "your mother and lover and god and compadre") whose mask finally drops at the Bottom to show the predator underneath — and the terror is that the predator's face is your own in the mirror. The Disease does not afflict a self; it replaces the self, "devoured or replaced and become you," so that by the end there is no person left who has an addiction, only the addiction wearing the person.