Nightmares

A big part of what makes the dangers lurking below the Surin Valley so terrifying is the way that it slowly but deliberately breaks and drives mad those who stray too far from the surface world. This is going to be represented throughout my adventure series in a number of different ways, but primarily in the Fortress of Bazarax, it will be through dreams (or more specifically, nightmares).

While I have plans for many of the nightmares the PCs could possibly have to foreshadow future developments, I'm wanting to have a pretty long list for DMs to choose from.

High fantasy heroes routinely face off against demons and monsters. What might terrify them while they dream?

Oh that's such an interesting idea! I love dreams/nightmares in tabletop games. It makes it so much more immersive.

I guess the scariest thing about nightmares is that they blur the line between real and imagined: while you dream you believe you are awake. So an "encounter" where the PC is facing off against a terrible foe - but they cannot draw their sword from its sheath, and no matter what they do their legs will not move faster than a walking pace - becomes scarier when its perceived as reality.

But as for the nightmares themselves, I think you're right - demons and monsters aren't that scary for heroes since they face them every day. Nightmares about things they don't have any control over are scariest: falling, being trapped etc.

Since my novel is comedy, my MC's dreams are usually strange or funny. As for nightmares, since he's not your typical fantasy hero, dreams about monsters would probably frighten him.