Once per a game, the auditor is allowed to kill a mafia member that isn't themselves and turn a villager into the mafia. Essentially a yak by proxy without having to kill themselves. They are sided with the mafia obviously.

The message people who get audited see is that they were selected to go on a private fishing trip. They receive this message the very moment they are selected and know they will die. The next day, they die and everyone doesn't know they got audited.