I found this awhile ago, and thought it was a really interesting look at what can make an effectively terrifying gaming experience:
One of the things I try to ascribe to when designing spooptacular levels is to try to keep players in the dark (mostly figuratively, sometimes literally...). At times, the scariest thing you can do is let the player's imagination run wild, as mentioned in the above video. The Cradle from TDS, for example, does this perfectly; the opening section has no enemies to speak of, but the creepy ambient sounds, atmosphere, worried commentary, and scripted events give us a taste of what we can expect while leaving us guessing as to what we'll eventually find ourselves up against. First playthrough, most players are likely psyching themselves out, wondering what horrible thing is waiting for them around the corner. Doing this in TDM, you'd probably have to keep a few things in mind. 1. Fear stems from powerlessness. The more in control you are, the less afraid you'll be. To branch off that, veteran players will likely be harder to scare than newer players, since they're the ones who know how things work (e.g. I can consistently dance rings around the zombies in Thief, since I'm now very familiar with their behavior). This is why the alien in Alien: Isolation works so well; because it adapts to your actions, even seasoned players never quite know what it's going to do, therefore making it completely unpredictable and bringing back that element of tension. I don't know that it's possible to vary TDM AI behavior like that, but I will say that implementing something new or very infrequently seen could be an effective strategy for scaring players. 2. Jumpscares can be effective, but are ultimately cheap. I'd say limit them to maybe one per mission, if even. Any more just gets old. 3. Gradually easing players into it might potentially be effective. My hypothesis is that slowly building up tension (e.g. through notes that get progressively more disturbing as you go) vs dumping it all on at once can cause that uneasy feeling to creep up on them unconsciously, but that could be up for debate. 4. Environmental storytelling will likely play a big part in setting the mood. Just setting up a small scene in a single room (e.g. the fireplace is all charred, the tools have been knocked over, the grate is crooked, the curtains over there are scorched, and there's a trail of ash and burnt charcoal leading to that door over yonder...) can build on the sense of wrongness and get the player asking "what happened here... or do I even want to know?". 5. Different things will scare different people. Some folks are total arachnophobes. Others really hate creepy dolls, or shadow people, or undead, etc. I don't know if it's worth playing up those fears or not (especially considering you have no idea who's going to play your mission, when, or with what kind of mindset), but possibly worth keeping in mind. 6. Really good sound design can make a huge difference. Definitely a no-brainer, but you don't want something like Yakety Sax playing in the background But at the end of the day, TDM is an immersive sim, which means there's a lot of moving parts to consider, from interest curves to core gameplay loops, that could tie into a good horror experience. Personally, I enjoy more subtle horror that ramps up over time, but there's a lot of wiggle room and many viable ways to design a good scary mission.