At one point in Dragons of Autumn Twilight, the heroes encounter a black dragon, and, under the influence of its fear aura, scatter like the comically low-level PCs that they are. Some of them make for a mysterious temple, in which they may face even more sinister threats.
Except they don’t. It turns out to be a Good temple filled with plot-advancing blessings. But there was an opportunity there to really put the screws to the players.
A dragon (or another monster, or a magical effect) that causes fear can be used to herd players in a direction that they really don’t want to go. Imagine if the heroes had peeked into the temple, and seen eldritch creepiness and wrongness of all sorts. As they try to leave, the dragon pops up. Characters who fail their saving throws are under movement constraints: their movement must take them farther from the dragon, if there is such a path available. Characters who pass their saving throw might still think it’s a good idea to move away from the dragon.
Movement away from the dragon inexorably draws the PCs closer to the entrance of the evil temple.
The really frustrating thing here is that the DM doesn’t move the PCs into the temple; they go themselves. Their options are limited to standing to face the dragon and entering the temple under their own power.
Here’s another fear-based DM trick inspired by the dragons of Dragons of Autumn Twilight:
So terrible, so agonized was the scream that Tanis dug his fingernails into his palms to keep from adding his own voice to that horrible wail and revealing himself to the dragon.
The PCs are hiding in the dark from a monster. (Maybe this is one of these encounters that is a little too tough to face head-on.) The monster has a fear-based attack that imposes penalties on the PCs: maybe attack penalties, maybe movement penalties.
The monster has another attack, used for locating cowering prey. It can attack the minds of anyone within a certain radius and make them scream in terror. It uses the scream to home in on its victim.
Here’s an odd note: in the original game module, we find this text: “The dragon wears a ring of darkness which projects up to a radius of 100′.” ON WHAT?