Then He will also say to those on His left, "Depart from Me, accursed ones, into the eternal fire which has been repared for the devil and his angels...And these will go away into eternal punishment, but the righteous into eternal life. (Matthew 25:41; 46)
To the lost who will be gathered on His left the King will say, "Depart from Me, accursed ones, into the eternal fire which has been prepared for the devil and his angels." Joining the unredeemable devil and his angels in the eternal fire of hell will be those human beings who refused to believe.
Jesus is speaking of eternal separation from God and from His goodness, righteousness, truth, joy, peace, and every other good thing. He is speaking of eternal association with the devil and his angels in the place of torment God prepared for them. He is speaking of eternal isolation, where there will be no fellowship, no consolation, and no encouragement. He is speaking of eternal duration and of eternal affliction, from which there will be no relief or respite.
Jesus uses the same word (aionios, eternal) to describe salvation and condemnation. If believers will be in heaven with God forever, the lost will be in hell with the devil forever.
Since the millennial kingdom will be worldwide, there will be no place on earth for the accursed to go. They will be slain on the spot and go immediately into the eternal punishment of hell, suffering permanent, everlasting crystallization of their state of spiritual death. At the end of the thousand years their bodies will be raised (cf. John 5:28--29), and they will again stand before God for final sentencing and final condemnation in bodies suited for hell's torments.
But the righteous will go away into eternal life, to spend all eternity glorified with their Lord and Savior. In marvelous contrast to the prospect of the accursed, at the end of the thousand-year earthly kingdom the righteous will discover that their eternal blessedness will only have begun.