Halloween horror one shot

I'm working on a Halloween horror one shot and could use some advice. What I'm thinking so far is the party will be part of a group of villagers, regular farmers, shop keepers...etc,  no armor, no weapons, no combat experience, just your average commoner with low stats. I will have multiple character sheets prepared for them, they will die and when they do they will take over another character, they have one objective, to survive the night. I'm thinking the group of 15-20 villagers arrive back at their village as night falls, to discover that everyone in the village is gone and they are going to start dropping like flies, I'll start with a few NPCs before moving on to the PCs. As I said they won't have weapons, but can arm themselves with whatever they find, pitchforks, shovels, candlesticks...etc, but won't be able to do much, except for running and hiding. What kind of monster(s) should I use? I want it to be something terrifying  and very hard to kill if not impossible, I'm thinking of maybe even having a group of soldiers or high level adventures show up mid game to help, only to be completely wiped out in seconds....so nothing like zombies or skeletons. Thanks in advance 

I'd suggest using a system designed for horror, rather than this makeshift implementation of 5e. Call of Chuthlu comes to mind but there are probably others. If the monster is completely unkillable (i.e. can TPK a group of high level adventurers) then it doesn't need a statblock, and if you want it to be scary you should homebrew it so the party have no idea what it is. Because the unknown is where you get true horror. You don't even need to have the monster roll things, as there is nothing that will break the horror vibe more than the monster rolling a bunch of 1s and 2s in a row. 

You'll also want some atmospheric music and scary noise for the monster to foretell its arrival, as anticipation is where you get true dread. 

Fraz-urb'luu 

He's scary and has ilusions to trick the characters.

But they'll get TPK for sure

There's a system for exactly this kind of game in Van Ritchen's called Survivors. I'd recommend checking it out. 

That book also has plenty of horrific monsters that you could use.