A mechanical process in nature experienced by those souls who within the allotted time fail to reach union with their inner divinity (i.e. known as self-realization, liberation, religare, yoga, moksha, etc). The Second Death is the complete dissolution of the ego (karma, defects, sins) in the infernal regions of nature, which after unimaginable quantities of suffering, proportional to the density of the psyche, in the end purifies the Essence (consciousness) so that it may try again to perfect itself and reach the union with the Being.
"He that overcometh (the sexual passion) shall inherit all things; and I will be his God (I will incarnate myself within him), and he shall be my son (because he is a Christified one), But the fearful (the tenebrous, cowards, unbelievers), and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death. (Revelation 21) This lake which burns with fire and brimstone is the lake of carnal passion. This lake is related with the lower animal depths of the human being and its atomic region is the abyss. The tenebrous slowly disintegrate themselves within the abyss until they die. This is the second death." - Samael Aun Weor, The Aquarian Message
"When the bridge called “Antakarana,” which communicates the divine triad with its “inferior essence”, is broken, the inferior essence (trapped into the ego) is left separated and is sunk into the abyss of destructive forces, where it (its ego) disintegrates little by little. This is the Second Death of which the Apocalypse speaks; this is the state of consciousness called “Avitchi.” - Samael Aun Weor, The Zodiacal Course
" The Second Death is really painful. The ego feels as if it has been divided in different parts, the fingers fall off, its arms, its legs. It suffers through a tremendous breakdown." - Samael Aun Weor, from the lecture The Mysteries of Life and Death