Here "Its quiver" represents the enemy nation's army, and the arrows in the quiver represent the soldiers in the army. (See: [[rc://en/ta/man/translate/figs-metonymy]])
An open tomb is one that is prepared to have many dead bodies put in it. The enemy army will kill many people. Alternate translation: "That nation's army will use their arrows and kill many people" or "Because of the arrows of its army, many people will die and be buried" (See: [[rc://en/ta/man/translate/figs-simile]])