The two phrases "when your days are fulfilled" and "go to your fathers" have similar meanings and are combined for emphasis. They both are polite ways to refer to death and dying. (See: [[rc://en/ta/man/translate/figs-parallelism]] and [[rc://en/ta/man/translate/figs-euphemism]])
The right to rule as king is described by the place where a king sits. AT: "I will make his rule over Israel to last forever" (See: [[rc://en/ta/man/translate/figs-metonymy]])