The phrase "lake of fire" is used four times in the King James Bible (Revelation 19:20, 20:10, 14, 15). Jesus used the constant flames in the Hinnom valley to symbolize the destructive nature of this fiery lake used to carry out God's judgment.
Sermon on the Mount
Jesus stated, in his famous Sermon on the Mount, "Do not be afraid of those (other humans) who kill the body, but do not have power to destroy the life; rather, fear Him Who has the power to destroy both life and body (their entire existence) in Gehenna (lake of fire)" (Matthew 5:28).
The lake is sometimes mentioned in conjunction with brimstone (Revelation 19:20, 20:10, 21:8), which is a highly combustible mineral that produces a suffocating smell. Its flames are referred to as unquenchable (Matthew 3:12), like a furnace (13:42) or oven (Malachi 4:1) and having an effect that is eternal (Matthew 25:41, 46).
What Happens In It?
The lake is a place where anything thrown into it is destroyed. The Bible states those thrown into it will be turned into ashes (Ezekiel 28:18, Malachi 4:3) and instantly consumed. Due to its destructive nature, the fire will likely be hotter than the hottest known star, named Wolf-Rayet 102, which is a scorching 210,000 kelvin (377,540 degrees Fahrenheit or 209,726 Celsius).
Who will receive the penalty of being taken out of existence? Demonic spirits are fully aware of the judgment that awaits them for their disobedience. For example, a demon recognized Jesus when he was giving a message in Capernaum. The evil spirit, no doubt surprised that God's Son was preaching to the person he had possessed, cries out, "Ah! What have we to do with You, Jesus, the Nazarene? Have You come to destroy us? . . ." (Mark 1:24, HBFV).
The Devil's Destiny
The destiny of Satan the devil, our adversary, is the lake of fire. Satan's fourth and final war to wrest control of the universe from God will take place at the end of Jesus' Millennial (1,000 year) reign on earth.
After his efforts, yet again, fail miserably he will grabbed and cast into the lake (Revelation 20:10). His self-centered life will end and he will never be remembered (see Isaiah 65:17). The Bible reiterates this fate of non-existence three times in two verses discussing the penalty Lucifer will ultimately receive (Ezekiel 28:18 - 19).
Incorrigibly wicked humans will also be perfectly annihilated in the flames of the lake. The end time Beast and False Prophet, two humans inspired and possessed by the devil, will be the first to receive God's judgment by fire. During Jesus' Second Coming they will be arrested and cast, alive, into this place of perfect destruction (Revelation 19:20).
God will also use the lake, before He creates a new heaven and earth where only righteousness dwells (Revelation 21:27), to deal with the issue of sin among humans. He will do this either as a part of what Scripture refers to as the second resurrection period or as a distinct third resurrection.
The Ultimate Penalty
All those humans who have committed the unpardonable sin will be brought back to life, judged, and executed. They are those who willfully and with full knowledge have hardened themselves to oppose righteousness. Their "reward" for rejecting God's perfect love will be to perish a second time, known as the second death, in the lake (Revelation 20:11 - 12, 15). Its flames will permanently end their lives without any possibility of a future resurrection.
The last thing thrown into the lake of fire will be death (1Corinthians 15:26, Revelation 20:14). God, after death itself is removed from occurring again, will also use flames to cleanse the entire universe (2Peter 3:11 - 13, see also Revelation 21). He will then create a new heaven and earth, as well as a New Jerusalem, where he can eternally enjoy his righteous children.