from the Secret History:
Once a year, on Hallows Eve, the walls between the reality of Earth and the nightmare of Underworld get thinner. Things that once preyed on our world look through and hunger. In Antiquity and the Dark Ages, they came through every year and were worshipped by pagans and witches as the Old Gods, or called Demons and hidden from in fortified churches.
In the 17th century, a secret society of both scientists and clergy, called the Illuminati, created rituals to hold them back, and thus was born centuries of peace and reason. The Illuminati thrived forcenturies, and then splintered but each faction kept to the mission, and then the factions died out without recruiting replacements, or forgetting their mission.
For the last 20 years, a single mad old Illuminati has guarded the world with his barely- remembered ritual, and things still slipped through and left madness and death in their wake. Last night, the old man died in his sleep, and the world is doomed.