The "vast image" that the speaker of this poem sees is described in the second stanza, and is clearly a terrifying sight for him. Now how this "image" is described:
a waste of desert sand;
A shape with lion body and the head of a man,
A gaze blank and pitiless as the sun,
Is moving its slow thighs, while all about it
Wind shadows of the indignant desert birds.
This monster is of course symbolic, and many critics believe that this creature stands for the fear that Yeats felt at the overthrow of Tsarist Russia and also the rise of Fascism in Europe. Such ideologies that threatened to overrule the importance of placing the needs and best interests of individuals were greatly feared by Yeats, and this exchange of the freedom of the individual for totalitarian regimes can be seen in this creature with its gaze "blank and pitiless as the sun." Yeats wrote this poem as a response to contemporary events that he felt greatly concerned by, and therefore the creature he describes can be seen as a clear symbol of what he feared. Such portents communicate in the "second coming," a new birth awaited at Bethlehem, but with a radically different result compared to the arrival of Jesus.