Appears in the words Jesus spoke to the church in Smyrna.
Rev 2:11He who has an ear, let him hear what the Spirit says to the churches. The one who conquers will not be hurt by the second death.
Souls who have not received salvation are all resurrected and judged again before the Lamb's throne. Being cast into the lake of fire at this time is called the second death.
Rev 20:11Then I saw a great white throne and him who was seated on it. From his presence earth and sky fled away, and no place was found for them.Rev 20:12And I saw the dead, great and small, standing before the throne, and books were opened. Then another book was opened, which is the book of life. And the dead were judged by what was written in the books, according to what they had done.Rev 20:13And the sea gave up the dead who were in it, Death and Hades gave up the dead who were in them, and they were judged, each one of them, according to what they had done.Rev 20:14Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire.Rev 20:15And if anyone's name was not found written in the book of life, he was thrown into the lake of fire.
Rev 21:8But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death.