Symbolism, Imagery, Allegory
At the very end of the book, White proclaims his ardent passion for death. Seriously—it's practically romantic for him. Check it out:
"I know what is out there and I know who is out there. I rush to nuzzle his bony cheek. No doubt he'll be surprised to find himself so cherished. And as I cling to his neck I will whisper in that dry and ancient ear: Here I am. Here I am. Now open the door." (141)
Of course, it's classic to portray death as a dark figure, a Grim Reaper or Angel of Death or someone else with a "bony cheek." In this case, White is rushing toward this being with whom he seems to feel a sense of kinship. He is, after all, "A professor of darkness, the night in day's clothing" (140)—so even if none of the people he was in group therapy with seemed like one of his "constituents" (as Black put it), the Grim Reaper evidently does. These two are kindred spirits it seems.