In Dune, it is established that De Vries had pioneered a type of toxin called "residual poison" which remains in the body for years and requires an antidote to be administered regularly. One such fatal poison is secretly administered by the Harkonnens to Thufir Hawat, the Mentat of House Atreides, in order to keep Hawat's allegiance as the only provider of the antidote (in the 1984 movie version, it is shown that Hawat has to milk a gruesome captive cat for the antidote every day).
De Vries is generally regarded as architect of the plan to destroy House Atreides, long-time enemy of the Harkonnens, while restoring the Baron's stewardship over the planet Arrakis. It was Piter's techniques and torture that broke Wellington Yueh, the Atreides Suk doctor's Imperial Conditioning against taking a life. Yueh eventually betrays House Atreides. Yueh gives the captured Leto a false tooth containing a poisonous gas. When the tooth is crushed, intended victim Baron Harkonnen escapes, but Leto and De Vries die.
Read more about this topic: Piter De Vries