Arkham Nights is a party game set in the world of H.P. Lovecraft. Players are assigned characters randomly and form two separate groups, the Townsfolk and the Investigator(s).
During the Day phase, each Investigator may ask one Townsfolk a question to help determine who is the hidden Cultist. The Investigator(s) may then choose to kill one Townsfolk if they wish. Some Townsfolk are Madmen who have an ability which is triggered when they are killed, others are Monsters who then challenge the Investigator who killed them.
Next is the Night phase, when the Investigator(s) leave the room to discuss what they have discovered during Day. Meanwhile, the Townsfolk determine how they can best help the Cultist succeed. Some Townsfolk are Madmen who have an ability which is triggered if they choose to permanently reveal their character. Finally, the Cultist chooses to either use a Relic, or remove a Ritual token from Cthulhu (represented by the Moderator).
If the final Ritual token is removed, the game ends with the Cultist and all the Townsfolk winning, living or dead. However, if the Investigator(s) managed to kill the Cultist during the previous Day phase, and the Cultist is still dead at the end of Night, the Investigators win and stop the ritual from completing.