From Arkham Horror Wiki
An Ancient One is a terrible and powerful alien creature from beyond time and space. In Arkham Horror the Ancient One is rousing from a long slumber, and it's up to the investigators to stop it from waking up! Each Ancient One sheet lists the powers, combat statistics, and worshippers associated with one of these alien beings. At the beginning of every game of Arkham Horror, the players randomly determine which Ancient One will threaten the city.
Ancient One mechanics
List of Ancient Ones
Epic Battles
