The version of this poem at The Eldritch Dark has a significant typo in the fifth line; the correct wording is shown below:
upon this sepulture adust and bare,
The poem's title is the Spanish word for "Nothing".
The theme of "Nada" could be expressed as the persistence of memory, even to the point that it becomes a curse for one who would rather forget. The closing sestet is practically a complete poem all by itself, with a dark music reminiscent of the verse of Edgar Allan Poe:
Oblivion's river flows in other lands
than this where memory feeds a mordant spring:
the walking dead beseech with parching hands
the cool, far shadow of the raven's wing;
and, leaning from the mouldered bed of lust,
love's skeleton writes Nada in the dust.
There is a deep, grim finality to these lines, an acceptance that something great has been lost. The love that was will never be again, and leaves little but bitterness and regret in its wake. Clark Ashton Smith was approaching the age of sixty when he wrote "Nada", and a lifetime of experience speaks boldly through these lines.