The end of the Millennium marks the beginning of what Bible scholars refer to as the “Eternal State,” which begins with a new heavens and new earth. Jesus will then turn everything over to the Father, according to 1 Corinthians 15:24-28:
Then comes the end, when He [Jesus] delivers up the kingdom to the God and Father, when He has abolished all rule and all authority and power. For He must reign until He has put all His enemies under His feet. The last enemy that will be abolished is death. For “He has put all things in subjection under His feet” [Ps. 8:6]. But when He says, “All things are put in subjection,” it is evident that He [the Father] is excepted who put all things in subjection to Him. And when all things are subjected to Him [the Father], then the Son Himself will also be subjected to the One who subjected all things to Him, that God may be all in all.
Satan, who had been bound for the duration of the thousand years, will be released at the end of the Millennium. He will then deceive those who are inwardly rebellious toward Jesus but who have been feigning obedience to Him (see Ps. 66:3).
God will permit Satan to deceive them in order to reveal the true condition of their hearts so that they can be rightfully judged. Under his deception, they will gather together to attack the holy city, Jerusalem, intending to overthrow the government of Jesus. The battle won’t last long because fire will come down from heaven to consume the surrounding armies, and Satan will be cast permanently into the lake of fire and brimstone (see Rev. 20:7-10).
That future gathering for battle is foretold in Psalm 2:
Why are the nations in an uproar, and the peoples devising a vain thing? The kings of the earth take their stand, and the rulers take counsel together against the Lord and against His Anointed [Christ]; “Let us tear their fetters apart, and cast away their cords from us!” He who sits in the heavens laughs, the Lord scoffs at them. Then He will speak to them in His anger and terrify them in His fury; “But as for Me, I have installed My King upon Zion, My Holy mountain.” “[Jesus now speaks] I will surely tell of the decree of the Lord; He said to Me, ‘Thou art My Son, today I have begotten Thee. Ask of Me, and I will surely give the nations as Thine inheritance, and the very ends of the earth as Thy possession. Thou shalt break them with a rod of iron, Thou shalt shatter them like earthenware.'” Now therefore, O kings, show discernment; take warning, O judges of the earth. Worship the Lord with reverence, and rejoice with trembling. Do homage to the Son, lest He become angry, and you perish in the way. For His wrath may soon be kindled. How blessed are all who take refuge in Him!