In line 11 of Shakespeare's Sonnet 18, death is personified as someone who can "brag" about the souls he has taken in death to the underworld similarly to how the god Hades takes souls to the underworld. We see the underworld being referred to in the phrase "wander'st in his shade," since shade refers to darkness and death and the underworld are very dark. We can particularly tell that death is being personified because the word death is capitalized, indicating Shakespeare is referring to a person, not just a concept. Plus, death cannot literally "brag"; only people can literally brag.
Essentially, Shakespeare is using line 11 to say that Death will not boast that he has captured the person being spoken of in the poem because Death will not be able to capture her--she will live on forever. Plus, the reason she will live on forever is because she is captured in Shakespeare's sonnet about her, and his sonnet will live on forever.