Desdemona is the victim of three powerful men in the play, Iago Roderigo, and Othello, but she is ultimately the victim of Iago.
When Iago sees Cassio flirting with Desdemona to congratulate her for insulting Iago who was insulting women in general, Iago quickly sees that he can use Cassio's flirtation against him to make him loose his lieutenancy (Act II, Scene 2). The direct victim of Iago's plan is Cassio, however, Iago has made it clear that he hates Othello and is probably willing to take down anyone associated with Othello in order to take revenge, even Desdemona. Thus, Desdemona becomes an indirect victim of Iago's plans.
Desdemona is also the victim of Roderigo in that Roderigo wanted to marry her instead and is willing to plot with Iago to try and break up the marriage. Iago uses Roderigo as an instrument to anger Cassio, whom Roderigo believes Desdemona may have an affair with, and start a fight that results in Cassio being stripped of his lieutenancy.
Finally, Desdemona is the victim of Othello who suffocates her because Iago has led him to believe that Desdemona is having an affair with Cassio. Iago set up the belief by first having Cassio ask Desdemona to plead Othello for Cassio's lieutenancy back, and secondly by planting Desdemona's handkerchief, a gift from Othello, in Cassio's room.