I found out two things from writing this scenario. First, writing a horror scenario that is not Lovecraftian horror was difficult for me. I've been writing Call of Cthulhu scenarios for so long that it was hard to change my mindset, even just slightly. In the end, (I think) I was able to do it.
The other thing I found out was that the Cthulhu Dark RPG is extremely flexible.
Initially, I wanted the scenario to be generic and without a rules system so GMs could pick it up and run it in any system they desired; Call of Cthulhu, Dread, Chill, FATE, and so on.
However, as I was writing the scenario, there was a nagging feeling I had with one part that I felt needed some type of mechanic; in this scenario it was spirits possessing NPCs. I kept looking at it from the perspective that I was the GM and wanted to run it - this was something missing that needed defined.
I decided I needed a system to put around it. After some soul searching, I chose Cthulhu Dark.
Cthulhu Dark is an RPG system written by Graham Walmsley that simplifies RPG to its essence - storytelling with very minimal mechanics. This fit perfectly into my scenario. I still wanted the players to have the ability to roll to do things, which Cthulhu Dark has. I still wanted some type of sanity system, which Cthulhu Dark has. But I still wanted it to be more story telling and roleplay, which Cthulhu Dark allows.
However, the possession mechanic that I wanted didn't really fit into the Cthulhu Dark rules. Then I found an article written by Graham on how to hack Cthulhu Dark. Following his advice on there, I created a new possession die used in my scenario.
The possession die is a d6 that represents a spirit's attempt to possess an NPC. When this happens, the die is rolled and compared to the group's Possession level. If the die is higher than the current Possession level, the NPC becomes possessed.
The group's Possession level starts at 4 and is decreased to 2 when a second NPC is possessed. In my scenario, this represents the various spirits getting more powerful with each possession.
In the end, the addition of the new die fit perfectly into my vision of the scenario and allowed me to do what I wanted. It also showed me the flexibility Cthulhu Dark gives a horror GM. Without something as simple as Cthulhu Dark, I'm not sure I would have been able to create what I wanted to create.
It also allowed me to create a scenario that has rules surrounding it, but can still be easily converted into any other system. Win, win!
Will I use Cthulhu Dark all the time? No. But its another tool in my toolbox that can be pulled out, and modified, to suit whatever needs I have. That is the real power of the system.