Those thrown into the lake of fire will be turned into ashes (Ezekiel 28:18, Malachi 4:3). In the HBFV translation of Matthew 5 it states Jesus used the constant flames in the valley of Hinnom (Gehenna) to symbolize the destructive nature of God's judgment upon all those who indulge in wickedness. He stated, "Do not be afraid of those (other humans) who kill the body, but do not have power to destroy the life; rather, fear Him (God) Who has the power to destroy both life and body (their entire existence) in Gehenna (lake of fire)" (Matthew 5:28).
Demonic spirits are fully aware of their fate, brought by fire, which awaits them. In what would become Christ's first recorded casting out of a demon, an evil spirit recognizes him during a synagogue service in Capernaum. The spirit, no doubt surprised that God's Son was preaching to the congregation, cries out in fear. He knows that Jesus has the power to destroy in him the fiery lake (Mark 1:24).
God declares, through the prophet Ezekiel, what will be the devil's ultimate destiny. He reinterates three times, in two verses, that the lake of fire ENDS the existence of anyone thrown in it (Ezekiel 28:18 - 19). Clearly, whatever God creates he can destroy. Its first use is at the time of Christ's Second Coming. The book of Revelation states it will be used to destroy the end time Beast and False Prophet who deceived the whole world (Revelation 19:20).
Its next use occurs shortly after the devil, at the end of Jesus' Millennial (1,000 year) reign on earth, will be set loose from prison and allowed to deceive the world one last time (Revelation 20:7 - 9). For at least the third time he will muster an army in order to wrest control of the universe away from the Eternal. His attempt, however, will utterly fail. He will be caught and tossed into the fiery lake where his life will end (Revelation 20:10).
Although the Bible is unclear regarding when demons will be judged, then receive their penalty, it will probably occur at the same time or shortly after the devil is put out of existence. Although the majority of them will likely be thrown into the lake of fire (see Luke 4:34), Scripture seems to indicate some will be restrained for all eternity (see our article on Satan's fate for more info). The book of Jude alludes to these two kinds of penalties (Jude 6, 7, 13).
God, before He creates a new heaven and earth where only righteousness dwells (Revelation 21:27), will also judge and execute all those humans who willfully reject his generous offer of salvation. This ultimate punishment in the Bible is called the second death, which will occur in the lake of fire (Revelation 20:11 - 12, 15).
The last thing thrown into the lake will be death itself (1Corinthians 15:26, Revelation 20:14). God, after death itself is taken out of existence, will use fire to cleanse the entire universe before he creates a new heaven, earth, and even a New Jerusalem (2Peter 3:11 - 13, see also Revelation 21).