Okay, I think I am a bit confused. So when exactly is the Horror Deck used? The rules you folks provided for download don't really go into specifics.
When is the Horror Deck used? The Horror Deck is used in any scenario that includes
horrific characters or
horrific encounters.
Normally, this is something you simply
choose ahead of time. For example, if I was playing the Lost Keys scenario from the basic book. I could
choose to make some of the characters on opposing side "horrific" simply because the figures look like monsters. And I might even make a couple plot points "horrific" to represent grisly murder scenes.
There is no rule controlling when you can do this. But we provide some different suggestions on the download, under the "Horrific Characters" section.
How do I know what is horrific? Before the start of the scenario, you simply decide who/what is going to be horrific. Use your own judgment. Note, in the
Tomb of the Serpent campaign, we identify the horrific characters and encounters for you.
I hope that helps. I don't mean to be vague, but a lot of it is really up to you to decide when you want to add a bit of
horror to a scenario.
Please let me know if you have other questions.