Here's a breakdown of the meaning:
* "Nor shall death brag thou wand'rest in his shade" means that death cannot boast about having taken the loved one, because their beauty will live on through the poem itself.
Here's why:
* "Death" is personified as a figure who takes pride in his power over life.
* "Wand'rest in his shade" means to be in the realm of death, to be deceased.
* "Brag" means to boast or show off.
* "Thou" refers to the beloved person.
Essentially, the speaker is saying that the beloved's beauty will transcend death, and even death itself cannot take away that beauty. The poem will act as a lasting monument to their memory.
The poem as a whole suggests that art and love can have an enduring power, even in the face of mortality.