Charles Ellicott Commentary Isaiah 66:16

"For by fire will Jehovah execute judgment, and by his sword, upon all flesh; and the slain of Jehovah shall be many." — Isaiah 66:16 (ASV)

Will the Lord plead ... —Better, will the Lord hold judgment. The thoughts of the seer pass on to the retributive side of the Divine righteousness. Fire and sword have been used by the enemies of God against His people, and shall, in turn, be the instruments of His vengeance. The “sword” may, however, be the symbol of the Divine judgment, apart from any reference to its human instrument (Deuteronomy 32:41; Revelation 1:16).