[Published with "Alastor", 1816.]
We are as clouds that veil the midnight moon;
How restlessly they speed, and gleam, and quiver,
Streaking the darkness radiantly!—yet soon
Night closes round, and they are lost for ever:
Or like forgotten lyres, whose dissonant strings
Give various response to each varying blast,
To whose frail frame no second motion brings
Quite creepy and dark to compare “we” (humankind, perhaps?) to dark clouds that suddenly vanish due to the forces of Time and Nature. This may be a metaphor for how death comes unexpectedly yet stealthily–and is permanent. 
 As stated  in the first line , humankind may be viewed as a menace towards Nature, but Nature gets her way at the end. 
 Also, notice how this stanza has a slant rhyme of “quiver” and “ever”– a deliberately jarring note in a poem of otherwise perfect ABAB rhymes in this poem.
Mutability