The third verse of the Isha Upanishad takes up the issue of the state after death based on the focus and action undertaken by the individual during his life:
“Sunless are those worlds and enveloped in blind gloom whereto all they in their passing hence resort who are slayers of their souls.”
There is a lot to consider in this verse, as we are introduced to concepts that our normal limitations to the surface mind and the outer life do not generally consider — the issue of other worlds, the issue of the determination of passing to worlds of light or darkness, the issue of ‘slaying of the soul’. We also see here what looks like a basis for the understanding of the action of the 3 gunas or qualities, sattva, rajas and tamas. Sattva represents light, tamas represents darkness. Rajas, when it embodies action filled with desire, leads to tamas or darkness. We see here, following up on the statements in verses 1 and 2, that action undertaken in consonance with the Divine’s will in the manifestation is recommended, and that action does not cleave to the soul. There is still the question however of action that conflicts with the Divine will in manifestation, either through what Sri Aurobindo calls “the materialist denial” with an overemphasis on the outer life and fulfillment of desires, or the “refusal of the ascetic” with its abandonment of the outer life. Either of these extremes, under the impulsion of rajas, can lead to darkness.
We do not usually consider what happens to the awareness, whether you call it a soul, or a stream of awareness or energy, after it departs from the life in the body. Yet the Upanishad makes it clear that it is a conscious Divine inhabitant of all the forms of the universe and thus, that consciousness takes new forms, but is not exterminated with the death of the body. The direction and focus of the energy in life leads to results; thus, those who focus on spiritual development, carrying out the Divine will in the manifestation would go to worlds of light, while those who wallow in the depths of desire and its siblings or children go to worlds of darkness. The Tibetan Book of the Dead describes after-death states (the ‘bardo’) which track the direction of the energy the soul has taken in life into worlds either blissful or dark and painful.
Sri Aurobindo notes: “By departing from the physical life one does not disappear out of the Movement, but only passes into some other general state of consciousness than the material universe. These states are either obscure or illuminated, some dark or sunless. By persisting in gross forms of ignorance, by coercing perversely the soul in its self-fulfilment or by a wrong dissolution of its becoming in the Movement, one enters into states of blind darkness, not into the worlds of light and of liberated and blissful being.”
Sri Aurobindo, The Upanishads, Isha Upanishad and analysis, pp. 19-20 & 27-33