Duke Leto Atreides, after he is captured, tries to kill his captor, Vladimir Harkonnen. What does he use?
Answer
A poison capsul hidden in his teeth
Duke Leto Atreides tries to kill Baron Harkonnen with a poison-gas capsule concealed in a false tooth.
The device is prepared by Wellington Yueh, Leto's personal physician, who has been coerced into betraying House Atreides. Yueh secretly replaces one of Leto's teeth with the capsule before delivering the drugged duke to the Harkonnens.
When Leto bites down on the tooth during his confrontation with the Baron, the released gas kills Leto and Piter de Vries. Harkonnen survives because his personal defensive shield helps protect him from the gas, although the attempt comes extremely close to succeeding.
A common mix-up is to call the device a poison pill. In the novel, it is specifically a poison-gas capsule hidden in a tooth; the tooth itself is the trigger mechanism. The scene also explains why Yueh's betrayal is more complicated than a simple act of treason: he believes the plan may let him strike back at the Harkonnens while creating an escape opportunity for Paul and Jessica.