Cthulhu and Sanity

With the inclusion of Insmouth in Ravenloft: The Horrors Within, why aren't D&D 5.5e rules for Sanity and Madness?

You might consider looking at the "Fear and Mental Stress" stuff in the 5.5e Dungeon Master's Guide.

There's also nothing stopping you from using the same optional rules for this that appeared in 5e.

In the 2014 Dungeon Master's Guide, there's a section on Madness that might be helpful if you have access. Also in Van Richten's Guide to Ravenloft there is a section about Fear and Stress.

In the Candela Obscura manual, there's a good section about sensitivity when it comes to using things like "sanity" and "madness" in games like this. People have real-world mental health issues, phobias, etc, and while creating or using rules to simulate some kind of "sanity bar" mechanic can sound interesting for a more horror-based or phycological-based campaign, it's important to, if you are going to do it, do it with respect to the actual real-life issues people face, and not turn it into some trope-ridden perversion of it.

Can you give examples?

Quote from RPG_Research >>

Can you give examples?

 

 Which part would you like examples of?

Just tell the players before hand if they would be ok with the Insanity rules.

If they can't play with those rules either they don't have to play or you don't have to use the rules.

Thus no one gets harmed.

Plus your not encouraged to act out all the insanity in public or even at all.
If my character is afraid of ghosts he gets the chance to roll a check against its effects. I don't have to run around the room screaming.

Rule number one above all else. Play the room. Don't worry about the people not in the room.

Don't gatekeep other peoples games with your phobias and fears.

Quote from TheGnome5786 >>

 

Don't gatekeep other peoples games with your phobias and fears.

 Is this is reference to something in this thread, or...

I don't think anyone is trying to tell anyone how they have to play or trying to "gatekeep"...