What major theme does the final couplet introduce into the sonnet?
love
death
life
old age

Old age is the major theme which the final couplet has introduced into the sonnet. A person should express his love in his life before the time comes to an end. The writer has strongly contemplated to spend life fully before the time checks out.

Explanation:

the answer is emerged from.

orpheus descended to hades, the world of the death to rescue eurydice. no mortal had ever done this before him, he survived because he was protected by the gods. the word "underworld" gives us a clue, he emerged from it like a whale emerges form sea for exemple.