This is the system I use in my Vampire: The Masquerade hack. It's so simple it feels almost unnecessary to share, but I think it works, especially for horror games.
Traits
Traits are words and phrases that define your mental, physical, social, etc. characteristics, both positive and negative. You can have any number of traits, but for each positive trait you must choose a somewhat equal negative trait. For example, you might be a professional dancer who studies medicine, but will bluescreen any device you touch and are always late.
Rolls
In a situation where the outcome isn't obvious, describe what you're trying to accomplish and the GM describes what could go wrong. Roll a d6, advantage/disadvantage from trait or situation if applicable:
- worst possible outcome (what the GM described happens)
- you can't get what you want
- a door closes, a window opens
- success with a cost
- you can't get everything you want
- best possible outcome (what you described happens)
Death
In mortal danger the worst possible outcome usually causes a lethal injury, killing you if untreated. You die from your second lethal injury. An overwhelmingly dangerous situation, such as fighting against multiple opponents, can kill with one bad roll.