"Ozymandias"

One major theme of Percy Bysshe Shelley's "Ozymandias" is that nothing lasts. The diction and irony of this poem each help to develop the theme that all earthly things shall decay.
The speakers' word choices portray Ozymandias as a once great, now forgotten, ruler. The traveler initially describes the statue of Ozymandias as "Half sunk," shattered," (line 4) "wrinkled," and "cold" (line 5), as though he were describing a corpse. It is eventually revealed that Ozymandias was once thought to be the "king of kings," so the use of such diction implies a great change in the esteem of this leader.
The words on the pedestal present such an irony from the initial description of the statue. One is led to believe from the first five lines of the poem that the statue was worthless, discarded without care. However, the inscription tells the reader that at one point, the mightiest of rulers feared this man. Yet at his death, all of his power was stripped away, and he is now but a forgotten statue, broken on the "lone and level sands" (line 14).
Thus, both the diction and irony used within the poem help the speaker to support the theme that nothing lasts on this earth.