Conclusion
The special purpose of this Upanishad is to give us the knowledge of the Real, so that we may not come under the dominion of the ego by identifying ourselves with our body, mind and senses.  Mortals become mortals because they fall under the sway of ego and depend on their own limited physical and mental strength. 

The lesson of the parable of the Devas and Brahman is that there is no real power, no real doer except God.  He is the eye of the eye, the ear of the ear; and eyes, ears, and all our faculties have no power independent of Him.  When we thus realize Him as the underlying Reality of our being, we transcend death and become immortal.

OM! PEACE! PEACE! PEACE!

Reference:
1. The Principal Upanishads - Dr S.Radhakrishnan
2. Kena Upanishad - Swami Lokeswarananda
3. The Upanishads - A New Translation - Swami Nikhilananda (in four volumes)
4. The Upanishads - Swami Paramananda

Also read other Upanishads
1. Mundaka Upanishad
2. Isha Upanishad
3. Suryopanishad