Once per round, when all impostors are dead, and there are [Set in settings] crewmates, there is a [Set in settings] chance that a traitor will spawn, and a random crewmate will be given a kill cooldown, that has no consequences, such as a normal impostor, The traitor wins with the impostor team, if they survive.
The traitor also can have a option to spawn with no NKS left, If people want to turn that on or not.