Kriminon

Kriminon was a poisonous gas capable of being carried in a tiny capsule, which upon its release might well prove lethal to the occupants of a large room, even an auditorium. It was mentioned in the Assassin's Handbook

Wellington Yueh armed Duke Leto I Atreides with a tooth containing kriminon. When Leto depressed the false tooth at his last audience with Baron Vladimir Harkonnen, the expanding cloud killed the baron's master assassin and very nearly brought about the baron's death.