When Nachiketa reached Yama’s palace his only desire was to know what is the absolute truth. The entire Katha Upanishad is based on the discourse.One section is very enlightening where Yama speaks aboutthe Hiranyagrabha. He says this is the place where all desires cease. Pain and pleasur no longer exist and there is fluidity. Yet even in this subtle space there is still  the distinction of the known and the unknown. There is something even beyond this ultimate. And Nachiketa persists. He asks for only that truth which is even beyond Hiranyagrbha. 

Yama laughs at the boy and is pleased because he knows the boy wants only the highest. But the reason Yama is not talking about this realm which is subtler than the subtle is because neither words , neither action can take you there. Nothing ccan describe it.  And it is important to remember even action cannot take you there. Rituals can take you to the space of Hiranyagrabha, but this space not so. Then how can one get there? He says it is only  the absolute who  can take you there.

And then he speaks of the one mantra that is the door way to this space. AUM. The only syllable that has the power to take you there. Merge in this space. Whilst chanting,in the right manner, he emphasizes on right chanting. Then you can merge with the mantra the deity and the worshiper becomeone.  The Absolute. But no one can describe it after all who will be left to describe it.