The narrator adopts an ironic attitude towards this mighty king for having been knocked off his pedestal--and through Ozymandias, sends a warning all tyrants. He feels a certain contempt for the bragging and egomania in Ozymandias.
The poem goes as follows: the narrator meets a traveller who has seen the statue of a once mighty ruler in the desert. Now it is just two legs standing up. The face, called the "visage," lies shattered on the ground, where the traveler see its "sneer of cold command." "Sneer" is an especially derogatory term.
On the base of the statue, the following words appear:
'My name is Ozymandias, king of kings: Look on my works, ye Mighty, and despair!'
But none of these works remain. The desert is completely empty, except for sand. The narrator is calling on "the mighty" to despair, because their delusions of grandeur will also be reduced to this. Like Ozymandias, none of them are as great as they think they are. In the end, they are empty braggarts.