14 • Then Death and Hades were thrown into the lake of fire.
• This is the second death, the lake of fire.
15 • And if anyone's name was not found written in the book of life,
he was thrown into the lake of fire.
Since we are incapable of seeing beneath the surface of a lake of fire so we can only get any idea of the horrors of its depths by comparison with known or imaginable things. For example the sea's flooding has a deadly tenacity but the flood will have been so much less tenacious than the fire will prove to be.
The lake of fire is called the second death because, since Death and Hades are thrown into it, there is to be no retreat back to the intermediate state of merely being dead. Hades is thrown in (and so too its space, so to speak, as well as its torments) to be assimilated into the lake of fire. If the first death is temporal (and indeed 'mortal' is in some ways synonymous with 'temporal') then when Death is thrown into the lake of fire its characteristic passage of time is also thrown into eternity and we may consider the second death to be a dying that never dies.
Those whose names are written in the book of life will not even taste the second death. Even now, for those who are 'asleep in Jesus', the torments of Hades are unknown because they and the pleasures they enjoy are separated from Hades by a great chasm that is fixed.
Although we can't look into the depths of the lake of fire the horrors of the road that leads inexorably there ought to be enough for us to determine not to go that route. We have much the same circumstance in reverse with wanting to be counted with those whose names are written in the book of life for we can't trace out those deeds done in the body with any certainty but certainty does come from getting to the bottom line so we simply ask, 'Is my name written in the book of life?'