As an investigator you solve mysteries in order to ban the Great Old One. A round begins with an action phase for traveling, resting, exchanging, preparing, looking for support and using „action“. In the encounter phase you fight monsters or resolve encounter cards and in the myth phase you resolve a myth card, including doom marker, revenge effects, appearance of gates, monsters and clues, displaying a rumor and myth markers and other events. When three mysteries and maybe the Last Mystery have been solved, investigators win, but there are several ways to lose the game.

The expansion introduces a new Great Old One, new myth cards, artefacts, supports and spells as well as new rules for combat encounters, alternate combat probations as well as the status of “lost in time and space”.