The Katha Upanishad is among the most popular of all Upanishads. It records an encounter between Naciketas and Yama (death) in which Yama reveals to Naciketas the mysteries of life after death. The conversation begins with Yama agreeing to grant Naciketas three wishes: (1) to see his dead father again, (2) to understand the ritual fire that leads one to immortality in the heavenly world, and finally, (3) knowldege of whether there is life after death. Yama resists granting this last wish, encouraging Naciketas to choose a host of worldly things: cattle, elephants, gold, horses, a magnificent house, etc. Naciketas turns it all down, choosing the preferable over the pleasurable.
Yama's answer is a clear outline of the doctrine of reincarnation and ultimate liberation, based on the identity of the universal and the individual soul. As long as the individual soul is bound to the world by desires and attachments, the soul is doomed to suffer an unending cycle of death and rebirth, leading only to death again and again; however, by employing the method of yoga one can suppress one's desires and attachments and walk the path to liberation. This is described in one of the Upanishads' most famous passages:
Arise ye! Awake ye!
Obtain your boons and understand them!
A sharpened edge of a razor, hard to traverse,
A difficult path is this -- poets declare!
By walking this path -- this razor's edge -- one can finally set aside the distraction of mortal life and understand the mystical doctrine that the individual soul is nothing other than the unborn and undying universal soul. Understanding this, one becomes free of the cycle of rebirth.
While not presenting the deep cosmological and theological insight of the Brihadaranyaka and the Chandogya Upanishads, the Katha Upanishad present perhaps the clearest eschatology of all the principle Upanishads. It is also noteworthy for its introduction of the method of yoga for achieving final liberation. Its brevity and depth recommend to the beginning student above all other Upanishads.