Dark Secrets is an expansion for Rise of Cthulhu. 

Dark Secrets adds all new Monsters, Artifacts, and Old Ones for players to use in their shadowy path to victory. In addition, the Expansion introduces a brand new mechanic: the Investigator. These “would-be heroes” set out to hunt down rumors of your corrupting influence and seek to stop you from awakening the Old Ones. When they appear, they force both you and your opponent to deal with the threat they bring, lest they halt the coming of the age of madness you seek to unleash.