A Poisoner is a killing role. Typically aligned with the Mafia, the Poisoner targets a player one night, poisoning them. The poisoned player then dies the following night without the Poisoner having to target them again. In this way the Poisoner can more effectively dodge Watchers and Trackers, by claiming "I didn't target that player the night they died!"
Typically Doctors can either not protect against a Poisoner kill at all, or can only do so the night the poisoned player dies, not the night they are poisoned. Although occasionally a Poisoner will be paired with an Antidote Doctor, who can cure poisonings whenever, but cannot protect normally.