Call of Cthulhu is the definitive horror RPG, published by Chaosium since 1981. Based on H.P. Lovecraft's Cthulhu Mythos, the game puts players in the role of ordinary people — professors, journalists, private investigators — who stumble upon cosmic horrors beyond human comprehension. The question isn't whether you'll defeat the monster; it's whether you'll survive with your sanity intact.
The system uses a percentile-based skill mechanic (d100). Your character has skills rated from 1-99, and you succeed by rolling under your skill value. It's intuitive and immediately transparent — a 65% in Library Use means you have a 65% chance of finding what you need. The Sanity mechanic is the game's signature innovation: encountering the supernatural erodes your mental stability, creating a tension between the need to investigate and the cost of knowing too much.
Call of Cthulhu excels at one-shot scenarios and short campaigns, making it perfect for convention play and pickup games. The 7th edition (current) modernized the rules while keeping the game's distinctive feel. Published scenarios like "The Haunting" and epic campaigns like "Masks of Nyarlathotep" are considered some of the best adventures ever written for any RPG system.