In Sonnet XVIII, Death is personified much like the Grim Reaper who comes for the beloved, desiring to claim her in "his shade"; this shade is an allusion to the valley of the shadow of death expressed in Psalm 23:4.
Written as a Petrarchan sonnet, Sonnet XVIII is composed of an octave (8 lines) that presents an argument, while the sestet (final 6 lines) offers an answer to this argument. The speaker argues that he does not wish to compare his love to a summer's day because of sublunary corruption; that is, things of nature deteriorate: "every fair from fair sometime declines." And, as a result of this deterioration, this sublunary corruption, the poet has decided to preserve his love and the object of his love in verse. With his "eternal lines," therefore, the beauty of his love will be everlasting, so long as men "can breathe, or eyes can see." Thus, Death and his "shade" will be thwarted, and he cannot brag of having taken the beloved, who will now live forever in verse where
...thy eternal summer shall not fade,
Nor lose possession of that fair thou owest;