How to run Temple of Empyreal Enlightenment to make it scary?

[Kauyon]

I read through ToEE and though I like the idea of a demon secretly corrupting an otherwise peaceful religious group, I'm having a hard time finding any actually creepy moments (aside from the suicide haunt, probably). I've seen a lot of people referencing this scenario as a really good horror adventure but I just have no idea how to creep my players out, how to deliver that feeling of "something is very, very wrong here". Ideally, I would want my players to totally lose their minds when they figure out what's really going on. Any advice on how to make this scenario scarier would be greatly appreciated!

[keerawa]

Assuming you are running the game remotely, prepare some widely divergent private messages based on characters saves, perception, and sense motive rolls. “The meal is simple, but delicious. You find yourself reaching for more.” “The food leaves an after-taste of rotting blood on the back of your tongue.” Things like that. If doing it in person, you can write out the messages on scraps of paper ahead of time.

The RP is very important. Prepare some catch phrases that will seem sweet or spiritual at first, but as the cultists repeat them, over and over, in an identical tone of voice when asked questions, it will creep your players out more and more.

[Matthew Downie]

Not specific to ToEE, but here's a pretty good old thread discussing of what makes an adventure scary.