This is the imaginary landscape where the great final simile of the poem comes to its catastrophic end. Just think about how far we've come in such a short poem, how far we are from the pleasure and calm of the beginning. We think there's something totally spine-chilling about the image of this pitch-dark battlefield.
Line 35: The image of the "darkling plain" opens up the epic simile (notice how he says "as on a darkling plain") that ends the poem. The basic idea here is that the speaker is comparing human existence to a dark battlefield, where friends and enemies clash together and fight each other in total confusion.