Iago is able to create gross and vivid images of Desdemona with Cassio which cause Othello such torment that he has a seizure. This is shocking for the audience as we see Othello lose both physical and mental control at the word of Iago who implies that Cassio will-
lie … with her, on her; what you will.
He convinces Othello that he will be able to overhear Cassio talking about his relationship with Desdemona. The audience is aware that Cassio is talking about Bianca, however, when he says how she dotes on him-
She was here even now; she haunts me in every place.
Iago has convinced Desdemona that she should appeal to Othello to reinstate Cassio. She is not aware of Othello’s suspicions of a union between her and Cassio, and unwittingly angers him until he strikes her .
At this point in the play, Othello has become convinced that Desdemona gave away his handkerchief. Iago pretends to defend her by saying her handkerchief is hers to give. He keeps reminding Othello of the token. Iago draws out Othello’s curiosity by saying that Cassio has bragged about his affairs. Then he frustrates Othello by not immediately telling him exactly what Cassio has admitted. The graphic suggestion that Cassio lies “With her [Desdemona], on her; what you will,” and that he undermines Othello’s reputation by telling people this, sends Othello into an epileptic seizure.
Iago talks to Cassio where Othello can see but not entirely hear the conversation. Iago assures Othello that Cassio is mocking Desdemona, when he is actually talking about his lover Bianca. Coincidentally, Bianca barges in and accuses Cassio of giving her “some minx's token,” Desdemona’s handkerchief. This makes Cassio look even worse—he got the handkerchief from a married woman and then, according to Iago, “hath given it his whore.” Whenever Othello leans towards leniency, Iago steers him away from the idea. Even poisoning the innocent Desdemona isn’t enough for Iago: “Do it not with poison, strangle her in her bed, even the bed she hath contaminated.”
Othello is so worked up about Cassio that he rages over an order for Cassio to take his place in Cyprus. He lashes out at Desdemona, who still speaks well of Cassio, thinking that she is sleeping with him. Because Iago is so close to Othello, Emilia and Desdemona turn to him for help. Iago feigns confusion, attempts to dispel the possibility that “Some base notorious knave, some scurvy fellow” has lied to Othello, and suggests that only “The business of the state does him offence.” It is the beginning of the end for everyone involved.