Katha-2-2-8-ya eṣa supteṣu

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

What was pledged in ‘I will explain the secret Brahman’ is now explained. This Purusha who wakes, i.e., does not sleep, when all, i.e., when prâna and the rest are asleep, creating by ignorance whatever was desired, i.e., objects of perception, such as woman and the rest, is certainly pure; and that is the Brahman. There is no other secret Brahman. It is that alone which is said to be immortal, i.e., deathless in all the sâstrâs. Moreover, the worlds, earth and the rest, all depend on the Brahman, that being the cause of all worlds. None passeth beyond that, etc., as already explained.

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