A metaphor is a figure of speech in which a writer compares two unrelated objects in order to paint a more vivid picture in a reader's mind. The use of a metaphor is especially helpful when a writer is trying to depict abstract concepts. Metaphors are also related to similes in that both forms of figurative language draw comparisons. The difference, however, is that a simile only draws a comparison by saying that something is like something through using either the words like or as. A metaphor, on the other hand, draws a comparison by saying that something is something else.
Dr. Wheeler gives us an example of a metaphor found in the first two lines of a hymn composed by Martin Luther: "A might fortress is our God, / a bulwark never failing" ("Literary Terms and Definitions: M"). Here, Luther refers to God as being a fortress, but since we know that God is not literally a fortress, we know this is a metaphor aimed at describing God as being strong, powerful, and protective, just like a fortress.
If a writer uses the clause, "IT is the devil," and does not mean to say that the devil has just literally walked into the room or made himself literally present in some other way, then we know the writer is comparing whatever is meant by the pronoun "IT" to the devil to say that "IT" is as evil, manipulative, and harmful as the devil. And, of course, the form of figurative language that draws comparisons between something to say that something is something else is called a metaphor.
We see a similar example of this metaphor being used in the Book of Matthew of the Bible (NIV). In Chapter 13, Verse 39, Jesus explains the meaning of a parable he relays earlier, and in one his explanatory sentences, he states, "the enemy who sows [the bad seed] is the devil." In one sense, he means this literally--the one who influences people to behave in ways contrary to the ways of God is literally the devil. However, in another sense, the devil does not literally walk about directly influencing people; instead, he uses the negative influences of others to spread his negative influences about the earth. Therefore, in another sense, Jesus is comparing peoples' enemies to the devil in order to describe them as having the same traits as the devil.
Hence, whether or not your clause in question truly is figurative language depends on if your writer is speaking literally or figuratively.