Two situations give rise to the infamous occurrences known as blood moons: astronomical alignments that result in a calendar month having a second full moon, and atmospheric phenomena when pollution and toxins hang heavy in the air, distorting the rising moon and giving it a blood-red cast and a seemingly larger size.
Both types of occurrences are considered bad omens, and are revered as unholy events by worshipers of deities of filth and decay. While the light of a blood moon shines, the DC to resist negative channeled energy increases by 2, and creatures exposed to its light take a –2 penalty on saving throws against diseases, curses, corruptions, and madnesses. Certain foul occult rituals related to such magic must be performed on blood moons.
Blood moons caused by astronomical alignments last as long as the moon is risen, while blood moons that result from atmospheric distortions last 1d4 hours, fading as the moon rises higher in the night sky and casts off its ruddy sheen.
Pathfinder Roleplaying Game Horror Adventures © 2016, Paizo Inc.; Authors: John Bennett, Clinton J. Boomer, Logan Bonner, Robert Brookes, Jason Bulmahn, Ross Byers, Jim Groves, Steven Helt, Thurston Hillman, Eric Hindley, Brandon Hodge, Mikko Kallio, Jason Nelson, Tom Phillips, Stephen Radney-MacFarland, Alistair Rigg, Alex Riggs, David N. Ross, F. Wesley Schneider, David Schwartz, Mark Seifter, and Linda Zayas-Palmer.