Katha-2-3-18-mṛtyuproktāṃ

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

This conclusion of the story intended to eulogise knowledge is now stated. Nachikêtas having obtained from Death by the granting of boons, the knowledge of the Brahman imparted as above said and also all the instruction about yôga with its fruits, attained Brahman, i.e., became immortal. How? Being already freed from vice and virtue and from desire and ignorance. Not merely Nachikêtas alone but even another who like Nachikêtas, knows the âtman and realizes its true nature unconditioned and within all, and not its form which is not within all. Who knows the nature of the âtman, as thus stated, such knower also; ‘being untainted becomes deathless, by attaining Brahman’ should be supplied to complete the sentence.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda