Sri Aurobindo translates Katha Upanishad, Second Cycle: Third Chapter, Verses 2-3: “All this universe of motion moves in the Prana and from the Prana also it proceeded: a mighty terror is He, yea, a thunderbolt uplifted. Who know Him, are the immortals. For fear of Him the Fire burns: for fear of Him the Sun gives heat: for fear of Him Indra and Vayu and Death hasten in their courses.”
The Prana referred to here is not the individual Prana, or breath, but the universal Prana, the cosmic breath, the cosmic motion that vibrates in the ether and brings forth the forms and dissolves them. The process of the manifestation involves the action of Prakriti, what is called elsewhere Para-Prakriti, the supreme Nature.
The Upanishads distinguish between the “knowers of Brahman” who are considered to be the “immortals” and the various separative forms and forces, including the universal forces such as fire, sun, wind and the action of death. The Kena Upanishad shows that the universal forces do not recognise the One, but attribute their power to themselves. The Taittiriya Upanishad emphasizes that recognizing even the slightest difference from the One is a cause for terror. The “immortals”, the “knowers of Brahman” do not have this fear as they see Oneness, but until that realisation is achieved, there is fear. Fear motivates movement or action, and thus, in the diversified forms of the universal creation, it is a power of putting the processes into motion.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129