(also known as Werewolf or Mafia)
Assassin is a great game - it's simple to setup, needs no equipment, and is heaps of fun. You need at least 7 or 8 people to play.
There are five roles people play in the game - The killers, a medic, townspeople, and a moderator. For a group of eleven, the ratios should be approximately:
Except for the Moderator, the other roles should be allocated secretly. Normally, the Moderator writes them on cards and hand them out, so no one knows what role others are playing. The moderator should be someone who knows the game well (perhaps a leader to start out with).
In a nutshell:
The game starts with the moderator telling everyone to close their eyes.
Once all eyes are closed, the moderator asks the Assassins to open their eyes. Given it's the first turn, each assassin will discover the fellow 'assassins'. The moderator asks the two Assassins who they wish to kill. They silently agree (by pointing to someone) who to 'kill' that turn. Once they have selected someone, the moderator thanks them and asks them to close their eyes.
The moderator then asks the Medic to open their eyes. They can then choose someone to 'save' (more on what this means later). Once they have selected someone to save, the moderator asks the Medic to close their eyes
Finally the moderator asks the Policeman to open his/her eyes, and select someone to accuse (of being an assassin). The moderator must honestly (and silently) answer the policeman with a nod or a shake of the head whether the accused is one of the assassins or not. The policeman then closes their eyes.
That's the end of the first round, and the moderator then asks everyone to open their eyes.
The moderator announces to the group who was killed during the night. The person who has been killed takes no further part in the game. It is essential they are not allowed to talk.
(Note that if the person the assassins selected to be 'killed' was also the one selected by the medic, the moderator only reveals there was a failed assassination attempt (and doesn't give any further information) so the person remains alive).
It is them up to the remaining group members to discuss amongst themselves who they think the assassins are. After a few minutes of discussion, the group must vote (one at a time) who they think is an assassin. Given the roles have been handed out in secret, the early rounds are often full of speculation, people justifying their innocence, or explaining their suspicions. And as the game progresses, lies, deception and strategy creep in. When all votes have been cast, the person with the most votes is out of the game (and must also not speak again). If there is a tied vote, give the group another minute for further discussion then vote again.
The policemen add a twist to the game - if they have accused someone and learnt they are an assassin, they may choose to disclose this to the group, but doing so has consequences - will the group believe they are the policemen? Often doing so puts a bigger target on their back - the assassins may choose to target that person next round. Or if the group are suspicious they may revolt and vote that person off.
The game continues on this cycle, with everyone closing their eyes, the assassins again selecting someone to kill, the medic selecting someone to save, the policeman someone to accuse, the moderator announcing who was killed, then voting again. If the medic and/or policeman have been voted out, the moderator should continue pretending to ask them to select someone to save (or to accuse) just so the townspeople don't know who those people are. If one of the two assassins is voted out, the game still continues with one assassin.
As the game progresses, less and less people are in the game, and people (especially the medic and policeman) build up more and more knowledge of who the assassins may be. Thus the discussions get more and more interesting. It's recommended the medics and policemen keep their identity a secret in the early rounds to avoid being picked off by the assassins. There comes a time however when it may be beneficial to reveal their role to the group, especially if the policeman has an accusation confirmed. Whether they can convince the rest of the group they are genuinely a policeman (and not an assassin posing as a policeman) adds to the mystery!
Adjust the number of policemen / assassins according to the size of the group. For 15 players, have two policemen, for 20, have three assassins, etc.