The answer is simple, really. It's because it represents their marriage. And it was carved into an olive tree, if I'm not mistaken. So if you wanted to move something carved into a tree, you would have to move the tree so... hope this helps in any way shape or form. :)
Your question is unclear or incomplete, can you give more details?
The figure of speech used in the lines from sonnet 10 might be personification, in this case, of the word Death. It might be said that personification is the figure of speech in which something nonhuman is given human characteristics. So the "Death" is firstly, personified by introducing it in capital letters, which means it is used as a name, a proper name. then it is given human characteristics, like "proud" "mighty" and "dreadful"
