Regarding Horror, you should definitely take what a lot of the previous posters have mentioned to heart. Enforcing stringent mechanics to batter your players with a 'Be afraid now!' stick, isn't likely to create a satisfying horror game, unless you /are/ looking for more of the Blood/Gore route - more on that in a bit.
A horror effect isn't going to be anything that you can create on your own, your players have to be on board and open to the experience. Much like the person two rows up from you in the movie theatre, who laughs through the entirety of a horror flick as a coping mechanism - a player who keeps busting one-liners is going to pull people out of the mood that you'll be trying to establish. As much of a throwaway term as it is now on the internet, immersion is going to be key. To get the most bang for your buck, you'll need players who are willing to embrace the weakness of the situation and feed off of it, you'll need players who allow themselves to care for the fate of NPCs, each other, and even the fates of their own characters. If your group is typically a die in a blaze of glory, shoot the Johnson, grab the cred, frag worrying about the alarms/police/security and screw any innocent chump who gets in the way - you're probably better off shooting for the hopeless unwinnable situation that evokes more of the 'How the frag we going to make it out of /this/ one?'
One suggestion that I have is to look into other horror RPGs for GM techniques, but keep in mind that stone cold mechanics alone aren't going to force the fear. Some games use Fear or Horror tests as a tool (Not talking about the Critter power
) - a threat to take the character's control away from the player in 'scary' situations. If all you do is apply this tool bluntly and repetetively, you're only going to frustrate your players. The key is to establish this test, this loss of player agency as a threat. Don't force anyone who roleplays fear to make these tests - reward them for good roleplay. Only hit the players who won't allow this 'weakness' with fear tests and the possible loss of control. After a while, you'll find the players who don't necessarily 'get into it' as much will start RPing hesitatance, or fear, even if it's a little tongue in cheek at the start, simply to avoid the mechanic. As soon as they start allowing this connection in-character the door opens to allowing themselves to feel the tension, and it's something that you can build on.
Strong horror is going to be a long play, one that depends on building connections and selling the atmosphere (as ScytheKnight said). You're going to have to provide NPCs that the players will care about and bank on corrupting innocence and betraying trusts. It's going to require a gaming setting without distractions like random people wandering through or portable electronics that will pull people out of the mood that you'll be trying to set and maintain. Jump scares aren't going to do it by themselves, but are occasionally usefull in a tense situation. Sound effects or tricks, such as subtly lowering your voice so that players need to lean in and focus on your narrative help a lot - look at videos of 'normal' storytellers telling scary stories, even the campfire ghost story kind - those will all show you techniques that you can use to sell it. When you're setting a particularly tense scene, imagine yourself as a predator - stand up and pace slowly around the table as you speak with a looming posture and narrowed eyes - you can tap into the psychological remnants of our distant past as prey to evoke a reaction. If you've already managed to build tension ... people /will/ get uncomfortable when you pass behind them and you move out of their vision. Especially if you pause to whisper something dread in their ear. Light, barely perceptible bodily contact like brushing hair - the type that gives people chills - can be used if you're in a comfortable group. A lot of this may seem like hamming it up, but it /is/ storytelling, in the end.
I know that this may be more technique focused instead of the story-building focused stuff that you might be looking for, so I'm not going to leave you empty handed. Here's an 'unwinnable/desperate situation' example. There's a mini-adventure in Ghost Cartels (4th Edition) that has the players go into Kowloon City in Hong Kong. It's a horrible environment in the fashion where humanity has no hope. It's dark, oppressive, and gives the feeling of having no escape. During the adventure, players are faced not only with a desperate defense scenario facing overwhelming numbers where they're forced to team up with 'enemies' to survive, but a wickedly strong (and _wrong feeling_) otherworldly threat as well. Check it out, and good luck!
- Wailer
