Mysterium is a board game published by Asmodee (in the US; your country may vary). It is essentially Dixit crossed with Clue. Quite literally, in fact, in the case of Dixit; the core cards in Dixit and Mysterium are of the same size and style, and may be interchanged and mixed as desired.
The story behind the game is that the players are clairvoyants brought in to solve a murder mystery. One player, in a role akin to a dungeon master, is the ghost, and sends clues to help the players guess the murderer. Unfortunately, the clues consist entirely of imaginatively illustrated cards, and the ghost is given a very limited set which which to communicate.
Each round the ghost sends 'visions' to the players, in the form of cards, and the players attempt to interpret them. The players receive cards indicating first the murderer, then the location, and then the murder weapon. This can be quite difficult to deduce, but if a player fails the ghost simply adds another card or two from their constantly replenished stock; even so, some of the players will deduce their targets largely through the process of elimination.
Players gain points based on how quickly they make guesses, and additionally from correctly predicting whether other players' guesses are correct or incorrect. However, this is essentially a cooperative game, and in the last round the players, as a group, get one last sending from the ghost which narrows the set of person/place/weapons from the range previously deduced by the players individually to a single guilty party. This last guess is a one-shot event, and the murderer is either found, for a successful game, or escapes, in which case everyone loses.
This is a fun game with the social component found in games like Apples to Apples added to a dose of good old-fashioned deduction. It is a challenging game for both the detectives and the ghost, so everyone enjoys the game. It can be played with anywhere from two to seven players (but it's probably best with four to five), and has good replayability. However, it does require some set-up, it requires that the ghost be knowledgeable about the rules, and it will take from 45-60 minutes for most games. It also costs ~$50.00 from most stores, although better deals might be found online. These factors mean that it is probably not a game for inexperienced board game players, but it is still a good choice for players who are looking for new and interesting games. If you get a chance to play a game with an experienced ghost who can walk you through the order of gameplay, I recommend that you take it!
Mysterium currently has one expansion, Mysterium: Hidden Signs, which adds new vision cards, suspects, locations, and weapons. As noted earlier, Dixit cards can be used as vision cards, and vice versa.