Sri Aurobindo translates Katha Upanishad, Second Cycle: Third Chapter, Verse 18: “Thus did Nachiketas with Death for his teacher win the God-knowledge: he learned likewise the whole ordinance of Yoga: thereafter he obtained God and became void of stain and void of death. So shall another be who comes likewise to the Science of the Spirit.”
The Katha Upanishad declares itself as a teaching for knowledge of God and the practice of Yoga. After obtaining this teaching, the seeker, Nachiketas was able to attain God-realisation and thereby overcome the control of death, through oneness with the Divine consciousness. This teaching is not limited to this particular individual, but is open to anyone who puts the teaching into practice. This is considered to be a “science” and thereby to be reproducible under similar conditions.
Death is indicated as the “teacher”. Anyone undertaking the spiritual quest eventually has to confront the significance of death. Wisdom traditions around the world take seekers through various ceremonies, rituals or practices that involve their putting themselves into a status of confronting death. The outer symbolic meaning of these rites translates to the inner realisation that the seeker must be able to face death.
It is also a reality for practitioners of Yoga that as they move inwards, abandoning the awareness of the outer world and entering into a state of Samadhi, or trance, that they have to abandon the ego-consciousness, which tends to react with fear of death. In some cases, this causes the seeker to draw back and return awareness to the outer consciousness. It is only after overcoming this fear, and giving up the attachment to the ego, and the desire-soul of the ego, that the link to other states of consciousness can be effected.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129