[RPG] How to make a Mi-Go city seem dangerous

First, here are some suggestions that don't involve hacking the rules.

1. Give NPCs one point of Health. This lets PCs kill lots of people easily, for full dramatic effect.

2. Make damage more interesting. For example, if the thing attacks you, you get infected. Alternatively, if you are hit successfully, you continue taking damage (because of, say, acid).

3. Work other rolls into combat. For example, have the Investigators fight a Flying Polyp: they must roll Sense Trouble to see it. Have them fight an aspect of Daoloth: they must roll Stability in order to attack it.

4. Calibrate combat so the Investigators are actually likely to fall below zero Health. They then need to make Consciousness rolls, which are a fun mechanic (they require spending more Health, which makes later rolls harder). Also, when they fall unconscious, other Investigators must run around helping them. This works best when fighting creatures that will kill you if you fall unconscious.

5. Use the rules in The Esoterror Fact Book. They let you take huge pools of Firearms and Scuffling, which you spend for benefits. For example, you can do a sniper-style Called Shot, to take out a particular opponent, by spending a few points.

None of this, however, fixes a particular problem with Gumshoe combat: you can always hit if you spend enough points. To fix that, you need to start hacking. Here is a small hack and a big one.

1. Let a rolled 1 mean an automatic fail. (This works well for Stability rolls, too.)

2. When you spend points of Firearms (or similar), they no longer give you points added to the roll. Instead, they give you extra dice to roll. For example, if you spend two points of Firearms, you roll the "basic" die, plus two extra. To hit, roll the Hit Threshold on any die.

This last hack, although it's a big one, has the pleasing effect that spending points makes you more likely, but never certain, to hit. Also, you get to roll handfuls of dice. You'll find the probability of hitting, when spending different numbers of points, is more or less the same.

Finally, here's a fun hack, stolen from Tunnels and Trolls.

1. Instead of rolling a damage die, let the damage equal the difference between your attack roll and mine. (Whoever rolls lowest takes the damage).

Trail of Cthulhu is based on the novels and the mythos inspired by H.P. Lovecraft's series of novels. I wouldn't spoil what the content of his novels specifically are, but in modern days it will be comparable with TV series such as X-Files and Supernatural, but in the 1930s.

In general, such games usually emphasis research and investigation, and tends to be gritty, though each group has it own lethality setting. Investigators are weaker than a 1st level character in any D&D game, and they do not just have to worry about their physical health, but their sanity as well. Indeed, a classic theme of the game is to trade your sanity for powerful occultic secrets that men are not intended to know. It is easy to get killed or to be driven insane, though that depends on the style of the game your group is running. You can run a Cthulhu-inspired game like a horror movie, or a with a more 'pulp' spin on it.

Though you did not ask for mechanics, I would like to mention one that makes Trail of Cthulhu unique. In the game you can always find core clues without rolling any skill, as long as you are trained in that area of investigation. You could ask to examine the body, and the GM will tell you any evidence you will need to further your investigation. Hence asking the right questions rather than "I search for anything unusual" is rather important.

Added on edit: Usually combat plays a smaller role in such games, and it will be esoteric knowledge, preparation and tactics that save the day, as the deck is usually stacked against you. Knowing what you are getting into and having the necessary protection against what you are up against is important.