Figurative language is any language used in a non-standard way to create non-literal meaning. Creating non-literal meaning can achieve certain effects. Some examples of figurative language are similes, metaphors, metonymies, oxymora, and many others. In his soliloquies at the end of Act 1, Scene 3 and Act 2, Scene 1, Iago uses a few types of figurative language to express his thoughts, his devious plan, and the reasons behind his devious plan, especially metaphors.
One example of a metaphor can be seen in his opening line in the soliloquy in Act 1, Scene 3, "Thus do I ever make my fool my purse." He is equating Roderigo, the fool, to his purse, saying that he will profit from Roderigo's stupidity. Another metaphor can be found in the line in which Iago uses the rumor that Othello has been sleeping with Iago's wife as an excuse to hate the Moor and break up the Moor's own marriage, as we see in the lines, "... And it is thought abroad that 'twixt my sheets / He's done my office." He continues using this as an excuse even in his soliloquy at the end of Act 2, Scene 1, claiming that he is seeking revenge because Othello slept with Iago's wife. In the Act 2, Scene 1 soliloquy, we see him use another metaphor to speak of the Moor's seduction of his wife, as we see in the phrase, "the lust Moor / Hath leaped into my seat." He even uses a simile to describe how angered he is at the thought of Othello sleeping with Iago's wife, saying that his anger is "like a poisonous mineral" gnawing at his insides.
Other interesting figurative language can be seen in his final lines of both soliloquies. In particular, in his Act 1, Scene 3 soliloquy, in the last two lines, "Hell and night / Must bring this monstrous birth to the world's light," the word "hell" is being used as a metonymy to mean the devil. A metonymy is a physical object that's actually being used to describe a more general concept, such as a crown can be used to express the idea of the monarchy and the White House can be used to refer to the Presidency. Likewise hell can be viewed as a physical place where the devil resides, so hell is embodying the devil. The phrase "monstrous birth" is further being used metaphorically to refer to Iago's evil plan, and the phrase "to the world's light" is a metaphor to simply mean to bring into sight or to bring into existence. In his Act 2, Scene 1 soliloquy, he uses a metaphor in his final two lines to say that his plan is still a little confused and that evil plans never really fall into place until the moment they're put into action. In particular, the line "knavery's plain face is never seen till used," meaning trickery's face is never seen, is a metaphor being used to refer to his evil plan and say that evil plans are never understood until being used.