The claim at the centre of chapter eight is stark. Whatever state a person calls to mind when they leave the body at the end, that state they go to, having been made to grow into it. It is not a rule about a final word or a deathbed formula. The chapter is explicit that the last thought is not chosen at the last moment; it is furnished by everything that came before, which is why the instruction that follows is to remember at all times, and fight, rather than to remember at the end.
The chapter then describes the departure in the technical language of the older tradition, with the breath held between the brows and the mind unmoving. It is describing a practice, not issuing one, and the atlas leaves it there. What follows is cosmology on a scale the rest of the book does not attempt. A day of the creator lasts a thousand ages and his night as long; at the coming of day all manifest things emerge from the unmanifest, and at the coming of night they dissolve back into it, the same multitude of beings arising helplessly again and again.
Beyond that alternation is an unmanifest that does not perish when all beings perish, called the highest goal, from which there is no returning. The distinction between an endlessly recycling cosmos and a state outside the cycle is the chapter's real payload.
It closes with the two paths, of fire and light and the bright half of the month leading on, and of smoke and night and the dark half leading to return. The imagery is inherited from the Upanishads, and the chapter presents it as knowledge a disciplined person should hold rather than as a claim to be argued.