I was angry with my friend;
I told my wrath, my wrath did end.
I was angry with my foe:
I told it not, my wrath did grow.

And I waterd it in fears,
Night & morning with my tears:
And I sunned it with smiles,
And with soft deceitful wiles.

And it grew both day and night.
Till it bore an apple bright.
And my foe beheld it shine,
And he knew that it was mine.

And into my garden stole,
When the night had veild the pole;
In the morning glad I see;
My foe outstretched beneath the tree.

—William Blake

As a literary device, an allegory is a metaphor in which a character, place or event is used to deliver a broader message about real-world issues and occurrences. Allegory (in the sense of the practice and use of allegorical devices and works) has occurred widely throughout history in all forms of art, largely because it can readily illustrate or convey complex ideas and concepts in ways that are comprehensible or striking to its viewers, readers, or listeners.

Writers or speakers typically use allegories as literary devices or as rhetorical devices that convey (semi-)hidden or complex meanings through symbolic figures, actions, imagery, or events, which together create the moral, spiritual, or political meaning the author wishes to convey. Many allegories use personifications of abstract concepts. (Wikipedia, s.v. “Allegory”)