Now when the thousand years have expired, Satan will be released from his prison and will go out to deceive the nations which are in the four corners of the earth, Gog and Magog, to gather them together to battle, whose number is as the sand of the sea. …
The devil, who deceived them, was cast into the lake of fire and brimstone where the beast and the false prophet are. And they will be tormented day and night forever and ever.
In Revelation 20:1-3 we see that Satan will be bound and kept from deceiving people for 1,000 years. (This is also pictured in the symbolism of the Day of Atonement in Leviticus 16:21-22). But he will be released and will again show his deceptive power, even on people who have lived under God’s beautiful Kingdom.
But finally Satan and his demons, as immortal spirit beings, will be locked away forever. The lake of fire will burn up those who are physical and mortal, but it is described as torment for those angelic beings who have permanently chosen the way of evil. Since physical fire cannot hurt spiritual beings, this torment must be mental torment.
For more about Satan’s fate, see “Satan Destroyed? How?”