It would be trite and not wholly accurate to say that no human pen could describe it, but one may properly say that it could not be vividly visualized by anyone whose ideas of aspect and contour are too closely bound up with the common life-forms of this planet and the known three dimensions.
Now this is what the rhetorical manuals call obscurum per obscures: describing something hard to represent in terms of something even harder to represent. One of Shelley's favorite tropes by the way, and mine too.
You can see how it relates to withdrawal and the way that (nevertheless) objects “sparkle” as he puts it: I think judging from a more recent post he's working on that very issue. There seems, to my too superficial eyes, to be something more disturbing than the normal frontal horror versus shadowy horror: a kind of chorismos between the two, an uneasy oscillation.