In the early days of Valyria the Blood Moon was a common natural phenomenon. Its blood red fog spreads across the land, causing death and despair in it's inhabitants. Terrified as they were, they had to find a solution. In their desperate situation they turned to the Ancients for help. They prayed to Umbra, the God of the Night. Hundreds of temples were built. Special offerings were placed in the temples dedicated to Umbra. The offerings consisted of man made things. In temples close to the sea they immolated sponges. In the desert they filled the temples with hay bales. The offerings in the temples of the prismarine woods were never recovered. They disappeared alongside with the temples and all signs of the original inhabitants.
Even years after this mystery the Blood Moon still happens every full moon. Your Nightmares and demons roam the streets, followed by the red fog.
Screams fill the blood red night. Demons lurking around your house, looking for a way in. Heavy breathing... Footsteps following you. Glowing eyes leering through your blinds. Waiting for the silence to start.
