Q:

What is the Five Layer Model of horror scenario design? 

I'm designing a horror part of my campaign.   I've heard of the Five Layer Model for horror scenario design but I'm not sure what it is.   Has anyone heard of it and if so can you either explain it or provide a link to it? 

A:

The Five Layer Model for horror scenario design is a handy tool put out by Graham Walmsley, as an aid for GMs to come up with good horror.  It's put out as a supplement to Cthulhu Dark, and as such, is really more of a guideline than a set of hard and fast rules. 
The rules themselves are in Dark Depths: Creating scenarios for Cthulhu Dark, and linked on the Cthulhu Dark tag page.  However, I'll summarize them here, as well as making some edits for readability. 

In short, the players should be drawn into the general mystery, and progress through the titular 5 increasingly unsettling layers during their investigation. 
The players should progress through each layer sequentially, and should encounter 2 events from each layer before unearthing the one beneath it.  Additionally, some layers have additional suggested requirements, made clear by their entries. 

For clarification, when reading through the layers, all ellipses (...) are intended to reference something from the layer below, even if that layer is not yet unearthed.   

Layer 1. 

Newspaper reports describing... 
A letter regarding...  
Folktales concerning...  
Someone who talks about...  

To unearth Stage 2, the Investigators must be
  somewhere that is at least two of the following:
  malodourous, unsettling, decaying, remote.   
Layer 2. 

Traces of the creature’s passing.   
Artifacts of unknown material and workmanship, 
  depicting grotesque creatures.   
Plants harmed by the creature.   
Indirect action against the Investigators.   
Folktales of...  
Someone who talks about...  
Warnings about...  

Layer 3. 

Warnings not to proceed further.   
Direct action against the Investigators.   
Ambiguous...  
Animal...  
Unnamed...  
Raving...  
Someone who talks about...  

Layer 4. 

Evidence of...
Victims of the creature. 
A glimpse of...  

To unearth Stage 5, the Investigators must be
  somewhere that is at least two of the following:
  dark, underground, ancient, alien.   
Layer 5. 

The creature itself. 
A named human harmed by the creature.