A land is reigned by a circle of powerful men who control every aspect of the citizen's life. This cabal changes members often. In reality, the spectres of a small necromantic covent possess and control the people in power. Since they simply possess the bodies, they can leave when old age overcomes their shells and possess a new up-and-coming noble.
Poison can only ever be the tool of a murderer - as such, alchemists are tightly regulated by the Crown, and anyone found in posession of an unknown substance is forced to taste it themselves.