Demand me nothing: what you know, you know: From this time forth I never will speak word. (5.2.11)
This is the last time Iago speaks in the play. After Othello demands to know why Iago set out to destroy him, Iago remains silent. But why? One would think that Iago would want to gloat but he refuses to explain his actions, leaving Othello and the audience pretty clueless about Iago's motives.