Hey Storytellers,
One way to amp up the suspense and terror is to put your players into a "closed-loop" scenario. If you ever watched any movies from the 1980s or before it was very common to have this happen to your heroes. There is a storm, the phone lines are cut (and cell phones don't exist yet), everybody is trapped in one location like a mansion or hotel. Queue lights turning off for less than a minute and then one person is killed. Queue lightning crash. Shocked gasps.
Ok, that is harder to do if your players have teleport or many forms of magic. Stuck in a room, "I cast stone shape." But here is a scenario where it works.
Your players come into a town. They can start to notice that fashion and language sound odd. A "knowledge Local" or similar knowledge will reveal the language is about 200ish years old. That is odd, but maybe the players ran into the RPG equivalent of the Amish.
I won't get into all of it here in the description, but we lay out many possible ways for the players to discover that there is a wizard who is going to perform a grand experiment in the next X days or hours. The issue is the experiment failed and it locked everyone in time.
Over the next hours or days, your players are slowly becoming a part of the story and are less and less able to affect it. If they can't stop the wizard from doing the experiment they will be trapped in the story forever. Of course, there are consequences to stopping the wizard.
This builds on the ideas of psychological horror and choice, as well as using a timeline to build suspense and terror. These three concepts were handled in earlier episodes not that long ago.