While it's technically a card game, this is more of a group mystery game. If you've ever played the game Assassin, you know how that works: a group of people, one the "killer". The goal was to find this killer. Heh. You don't know how easy you had it then...here's how it works:
First, you need a bunch of people. (10 is okay; 13 works best). One of these people is the narrator. Preferably someone who knows the rules, as they can explain things. Everyone else is playing.
The remainder of these directions will be directed at the narrator, just for fun. Find yourself a normal deck of cards. Take out a number of spade cards (doesn't matter which) equal to the number of people playing, divided by 3 (round down). Also take out the king of hearts, and enough other heart cards (again, doesn't matter which) to make up enough for the rest of the people (lessee....number of people playing, -1, -# of spades cards taken out).
As narrator, explain the rules. Everybody will be dealt a card (random or selected, narrator's choice; if you know the people, then it's fun to pick who gets what. If not, random is fine.) If you get a spade, then you are a member of the mafia. If you get the king of hearts, you are the Chief of Police. If you get any other heart, you are a citizen.
Now, everybody (except the narrator; in the future this will be assumed, and I won't have to say it) closes their eyes, and the narrator distributes cards. When you're done dealing them, tell everyone to open their eyes, look at their cards, and keep them secret from everyone else for the rest of the game. This is important. Players should only see their own card. You should also know who is mafia, and who the chief of police is. It makes things a lot easier. Now the game begins.
1) Now, everyone closes their eyes again. Tell the mafia members to open their eyes. They get to choose someone to die (it has to be unanimous, but it's a smart idea not to talk). When they're done selecting, tell them to close their eyes. Then reveal who has died, take their card, and tell everyone what they were (mafia, citizen, or chief of police; don't worry, the mafia usually isn't unanimous in killing one of their own...7;-)
This is important. When someone is dead, they can't say anything more relating to the case. However, they do get to keep their eyes open at all times, thus seeing who's mafia, etc. But no saying anything. (Or pointing, etc. Blah blah blah)
2) Next, everyone closes their eyes again (technically they should still be closed, but some people tend to open them when they hear who's dead. 7;-) Tell the chief of police to open his/her eyes. The chief of police then indicates a person to you, the narrator. (Again, it's a good idea to be quiet, here.) Say, out loud, (after taking their card, if necessary) whether that person is a member of the mafia, or an innocent. Tell the chief of police to close his/her eyes.
3) This is the fun part. Everyone opens their eyes. Now, the people still alive get to vote on who to kill. Since nobody (except the mafia members) know who is mafia, and who isn't, this is where lots of discussion, hearsay (literally), and paranoia (you *don't* want to kill him? You must *both* be mafia members!) come into effect. Normally, only one person is killed. It must be unanimous (except for the person to be killed, naturally). However, is special cases (mostly due to the fact that mafia members will tend to stick up for other mafia members), it is possible to kill more than one person. This is up to the narrator. When the decision is made as to who to kill, that person is dead. Take their card, and tell everyone whether they were mafia, chief of police, or a citizen.
Repeat as above, until either everyone alive is a citizen or chief of police, or everyone alive is mafia. The citizens win if they kill all the mafia members (though they've probably suffered a number of losses in the process...), while the mafia wins if they kill all the citizens and the chief of police.
Notes: