who take up the sword

The word "sword" is a metonym for the act of killing someone with a sword. The implied information can be stated explicitly. AT: "who pick up a sword to kill others" or "who want to kill other people" (See: rc://en/ta/man/translate/figs-metonymy and rc://en/ta/man/translate/figs-explicit)

sword will perish by the sword

"sword will die by means of the sword" or "sword—it is with the sword that someone will kill them"