Invention

In the final chapter of Brodmaw Bay there is a sentence describing my main character's final experience at a neolithic site above the village, a place by this point ominously familiar to him. It ends '... the looming stones screamed dumb questions about themselves to which time had forgotten the answer.'
Anyone who has been to one of those sites will be familiar with that feeling. They were built at a time so remote to us that we know almost nothing about them, or their purpose, or the people who constructed them.
This gives a writer the opportunity to invent. If you write about vampires or zombies, you are stuck with a pretty rigid mythos. The same is true of Satanism. Combating the devil requires soldiers of God.
But if you are dealing with beliefs and rituals from the west of England 5000 years ago (when Stonehenge was begun), you are obliged to make them up.
There are rules, obviously. The myths have to be plausible and resonate in the readers' imagination. They need to be culturally apt in a way that makes them seem almost familiar (in that insidious way that legends are taken for truth). And it helps if they are frightening, or at least unsettling in a way that makes them memorable.
Some writers create an entire mythos with a complex architecture. I have just conjured some Cornish coastal demons. But by the final page of the concluding chapter, I'm pretty sure Brodmaw Bay is not a place you'd really want to visit. You certainly wouldn't wish to live there.