Horrific imagery forms this poem's backbone. By submerging readers in a nightmarish vision of a world without a sun, the speaker insists on a confrontation with humanity's dark side.
Much of the imagery here evokes the sheer physical terror of complete darkness. Take these early lines, for example:
The bright sun was extinguish'd, and the stars
Did wander darkling in the eternal space,
Rayless, and pathless, and the icy earth
Swung blind and blackening in the moonless air;
The images of the dark here present it as an assault on the senses of both sight and touch: the earth without the sun becomes both "icy" and "blind," frozen and groping through the void of space. And the word "bright" in line 2 is the only truly luminous word in the whole poem: all the later fires only "blaze" and "flash" briefly before fading into dull embers.
Imagery also won't let readers look away from the misery of the survivors on the lightless earth:
[...] the wild birds shriek'd
And, terrified, did flutter on the ground,
And flap their useless wings [...]
The imagery here (underlined by the onomatopoeia of "shriek'd," "flutter," and "flap") stabs readers with pity for the helpless, panicking birds. It's as if, everywhere one turns, one can hear the screams of dying animals.
And by the end of the poem, when all life is finally extinguished, the speaker's imagery evokes complete nothingness. The world becomes a mere "lump of death—a chaos of hard clay." That "hard clay" feels cold, unforgiving, and deadly; speakers can almost feel the impact as this lifeless "lump" hits the poem.
All this imagery doesn't just help readers to feel what it would be like to live in a dying world. It also underlines the poem's bigger metaphorical point: that the inner "darkness" of evil, selfishness, greed, and despair are never really that far from the surface. Imagery makes the consequences of human evil concrete.