He convinces Roderigo to fight with Cassio in order to get Cassio in trouble.
Iago gets Cassio drunk, watches him get into a brawl, and then tells Othello all about it. After Cassio is fired, Iago suggests to him that he ask Desdemona to intervene with Othello in his favor. Cassio thinks this is a great idea. Iago gloats about how well his plan is working. Iago makes sure his wife will bring Cassio in to talk with Desdemona.Iago and Othello tour the city. Iago begins with small comments and gradually reels Othello in until he tells him point blank that Desdemona is cheating on him. Othello demands proof and Iago promises to give him some. Then Iago swears that he will help Othello revenge himself on Desdemona. Iago comes in with Cassio and finds Desdemona upset at Othello's weird behavior. He promises her he will find out what's wrong Iago stages a conversation with Cassio such that Othello, overhearing, thinks they're talking about Desdemona. In fact, they're talking about a prostitute (Bianca) who is in love with Cassio. Iago convinces Roderigo that he should kill Cassio.Iago watches Roderigo try to kill Cassio. Then Iago stabs Cassio from behind. When more of Othello's soldiers appear, Iago stabs Roderigo too, as if trying to help