When the first body killed, the killer must self report it and tell everyone that they are the impostor(This person will be called the Traitor). The thing is, they can not be voted off until the other impostor is voted off. They can kill/vent in front of other people if they want but they can not get voted off until the other impostor is found. The traitor can even spread misinformation of who the other impostor is. Once the other impostor is voted off, the crewmates are now allowed to vote off the traitor, and crewmates win.
image is not mine nor did i take this
