Interesting question.
I guess an important aspect is a good mixture of surprise (i.e. shock) and anticipation. Surprises are probably the actual scary part of most horror games or movies (unless they're more "psychological" so to say, such as David Lynch's movies or Donnie Darko, which mostly don't contain any "jump moments", just this scary sort of atmosphere), including the slender man you mentioned. Anticipation might seem to counteract those surprise moments at first, but I'm pretty sure in certain situations it pays off. As a good example, there is a game somewhat similar to Slender which is called SCP-087. All you do in this game is walking down stairs, and you know that you are going to die sooner or later. You just don't know when and how - there are actually very few shock moments, the game mostly works due to the player's anticipation of what might be going to happen.
In Half Life 2 (
minor spoilers ahead) there was the Ravenholm-level. A dark town full of zombies. At one point the fast zombies are introduced. They are able to run, jump and climb, they are very unpredictable and suddenly your initial supremacy over the dumb, sort of immobile zombies is gone. The scene I'm refering to can be seen in this video:
https://www.youtube.com/watch?v=fWN-GvixIto beginning at 2:30, but the best moment imho starts at 4:50. You just know, as soon as you call this elevator the zombies will come, then you start hearing them climb up the water pipes, and all you can do is wait for them, constantly aiming at the door.
(
Spoilers are over)
Apart from that, sound is certainly highly important.
A friend of mine made a game for Ludum Dare a few weeks ago - it's really small, the gameplay is rather bland and the game lacks polishing, but what he
did accomplish was a really good atmosphere due to good use of sound.
(Link) It just gives you this creepy feeling of absolute isolation and hopelessness.
Another effective method is causing
panic in the player. As far as I know, when the slender man sees you you have to run away quickly since you die otherwise. I think it would work even better if you didn't just loose the game, but the slender man actually
followed you. This panic-escape-reaction is used rather frequently in Amnesia, as far as I remember. (Haven't really played either of those games tbh)
Quote: "What type(s) of character(s) scare the living s**t out of you?"
Hard to say. Mostly humanoid ones I guess, which includes zombies, ghosts and actual humans.
Quote: "Single player or multiplayer (perhaps having player 1 play the protagonist and player 2 the antagonist)"
I haven't heared of any actually scary multiplayer title. And I don't really know how you'd do that. Might be an interesting experiment, but SP seems to work quite well for horror games.
Quote: "Would you use one antagonist or several?"
That *highly* depends on what kind of game you're making. No general answer to that. Using just one, as in Slender, can work well, but so does the presense of multiple antagonists. Using just one could of course be a more personal solution and a way to further immerse the player by building some kind of relationship. For instance, the player could be a man who's haunted by his crazy dead ex-wife or something.
Quote: "What type of a story line draws players in, keeps you interested and most importantly causes nightmares even after you're done playing for the night?"
What keeps me personally interested is mostly the use of interesting characters. This could be realized by a huge transformation of the protagonist, a deep relationship between protagonist and antagonist, or simply (preferably unpredictable) antagonists who communicate with the player and reveal their motivation. But there are surely many more ways to achieve that.
About nightmares... well, I guess the game just has to be really immersive. As soon as I actually identify with the player, the scares stop to be a mere form of entertainment (as in Paranormal Activity, I thought it was pretty scary, but as soon as I went out of cinema the experience was over) and have actual effects on me.
Quote: "What would the objectives be to keep with the story line?"
Nothing in particular. There are endless possibilities. Survive would be one, obviously. But it might be interesting to invert this premise and make dying an objective instead. However, surviving probably works pretty well since people can identify with that goal and it allows them to rely on their instincts without overthinking their actions.
