This kind of rebuking is also called a pleading with them; so he threatens to plead against Gog with pestilence and blood, Ezekiel 38:22. God at first pleads with sinners by words; but if he cannot so prevail, he will plead with them in a way by which he will overcome by fire, pestilence, and blood. With all flesh; thus he threatens to do with all sinners, or with all the wicked Jews. The slain of the Lord shall be many; to the fire he threatens to add the sword, so as the slain of the Lord, that is, those whom God should cause to be slain, should be many.
Related Commentaries of Isaiah 66:16
Isaiah 66:16
16 For by fire and by his sword will the LORD plead with all flesh: and the slain of the LORD shall be many.