The Elder Sign, perhaps invented by the Elder Gods themselves, is a symbol that can be written upon the ground or wall. It consists of a five-point star with a flaming eye within its center. Elder Signs may be written with chalk, ink, blood, or whatever may make a mark, even a carving will prove effective.
Drawing an Elder Sign drives away all beings with the "Eldritch" marker, making them an effective and very easy way to make an area safe from monsters. However, making one Elder Sign costs 1D6 sanity, and will only last as long as the marking, meaning even rain is enough to dispell an Elder Sign. Along with that, each Elder Sign takes one In-Game hour to draw.
The Elder Sign, if drawn on a person will force the person to seek out information about the Mythos, often causing them insanity.