/lit/ - Literature

Hey /lit/ need some writing advice for horror fiction.
I'm GMing a game tonight for a smallish group of 5 players. The mission I've got prepared is my first attempt at horror. The players need to go into an abandoned facility to retrieve something.
Give me advice to help write horror and high tension.
So far I've got:
- Time constraint
- Limited space
- extremely limited knowledge
- separate the party to be alone
- The monster can't really be fought & is very unknown. it will obscure senses when close to keep it unknown
- Their characters abilities and stats are nerfed as they get closer to the monster (making them helpless)
- I'm going to try to write with focus on atmosphere & feeling.

What else?