Question: In a horror themed campaign with a Lovecraftian feeling, a big part of the game is a constant feeling of threat, but without any visual stimuli other than glimpses, or sounds vaguely heard of things seething in darkness, and with investigation a big part of the game, contrary to fighting.

Having read *Heroes of Horror*, I am searching for ideas on how to actually motivate players to assume the role of investigator rather than executioner, and how to instil the horror of the unknown, of something that threatens but is not substantial, that cannot be seen or heard, but is there, and has malevolent intent.

I am thinking of D&D 3.5 as a system, but I am interested in answers that cover any system (Call of Cthulhu is unfortunately out of the question), more as role-playing and storytelling techniques rather than mechanical ones.

Answer: 
The biggest ally in the bid to creep the players out is their own imagination. The biggest obstacle to that is your own. When running these types of campaigns, less is always more. It's just about finding out what 'less' should include.

1. Descriptions should hinge around one detail- the players' imaginations should do the rest. It's not about gore, nor about ultra-detailed descriptions, it's about triggering the players' mind to go overtime. Hint at the unknown rather than assaulting the player with it.
2. Engage all of the players' senses. In fact, engage other senses more than sight; reducing the applicability of sight to the description helps to engage the imagination, i.e the strange coppery smell, or the vision obscuring smoke that smells/tastes faintly of burning meat.
3. Building upon the details with small incremental revelations helps to lead the players' to where you want them to be. The realization that something is missing, or that something has been lost after an encounter with the unknown makes it that much more disturbing.
4. Repetition helps to inure the players to what you're doing- especially if that repetition is rhythmic or predictable. Even if not predictable, there is definitely a short shelf-life for when it will no longer be effective.
5. It is said that the best lies blend a kernel of the truth. It is also true that the best horror blends elements of the every day or of innocence. Take something that is safe and everyday/innocent and twist it for an effective factor of creepiness, i.e. the evil prophesy delivered in a child's singsong.

There are gimmicky techniques that can be blended with these to make them more effective, i.e. the soundtrack that comes on at an unexpected time, turning the lights down/off, sending notes to players in a way that others can see, but forbidding the player to talk about what's on the note... but these are just gimmicks and are not necessary to invoke that feeling of horror. Engaging the players' imagination is the key.
**Cthonic entities always have a bigger hammer**

The essence of a D&D game nominally goes: "Kill things to get loot to kill things to get better loot to get levels to kill things." Part of this philosophy is that the heroes will be going up against things that they have a chance of killing.

Remember the ancient quote: "Cthulhu, driving 1d4 investigators per round insane." In a horror game, the enemy has an overwhelmingly bigger hammer. Direct confrontation, even in the most epic fights, will lead to player death and high odds of a total party kill. Make it quite clear to your players that you won't be pulling punches. 

The second thing to do is to play in "Theatre of the Mind" (taken from Monte Cook talking about D&D Next of all things). Battlemaps give concrete control of the world over to an objective reality. While this is good for tactical games, giving players this kind of knowledge gives them the impression that they'll be able to succeed in a combat. If you play mostly in theatre of the mind, you'll be far better off when combat does happen.

For that matter: never ever describe the ultimate BBEGs. The unknown is always scarier than the known. Players should engage with its cultists and hints of the BBEG, rather than the thing itself. (Contrast: Lovecraft with Ringo's Princess of Wands.) 

By making it clear that the players need to engage guile and subtlety because they lack a bigger hammer and by letting most of the game be imagination-driven (have props if you want, mind. A nice leatherbound tome is a fantastic player-trap. Anyone who is silly enough to read the book has his character driven insane. 

Hints, allusion, and cowering like roaches against unknown entities who may accidentally step on them. Your players are roaches who are unable to understand the sentences acting for their own ends. Players can strike against their cultists, not the entities themselves. At the same time, make it clear that the players can be hugely effective against the cultists... if only they know where to strike. Combats are easy, finding the targets is hard.

The threat is simple. Unfortunate coincidences are the act of conspiracy. Cultists love conspiracies and are working to achieve their own ends. When their objectives intersect the players, the players may suspect that there are ulterior motives at work. Otherwise... well, stuff just keeps getting worse as the cults get closer to their win state: "May I be eaten first"