also, spooky sounds and other things like that are the ONLY way to get this to be 'scary', try getting actual alien soundtrack to loop and various other sounds that play at random or when a player walks by to keep them on edge, the same for the alien which would activate these sounds to give the humans a 'warning' or location of the alien, making the alien play careful and sneakily rather then an extraterrestrial meat grinder
It is true that sound design is one of the most substantial pieces of a horror atmosphere. It doesn't need to be generally "Scary" sounds, just background noise placed in such a way to unnerve the player. Mechanical whirring, fluorescent lights buzzing, steam pipes leaking, metallic scraping, nearby footsteps all work. You want to keep sounds subtle, that way players start to get used to the background noise, and then make it quiet when threat is nearby.
You want to achieve a certain kind of "threatening" environment in order to put your players one edge. You want to make your players feel like their character is in danger, while giving them a reason to not just up and leave the server.
The two biggest issues with horror themed servers is that the anxiety and tedium can wear on a player if there's no other value beyond being scared. Keep a balance of terror and fun. Make it exciting, not just terrifying. Your server is basically supposed to be like a thrill ride; everyone's coming to experience your server.
Just jossing ideas, but you could have like a "safe" period before the alien starts to stalk the players where everything is bright and normal. Then, after the start of the round is over, the lights go out and people start getting picked off. You could have the build set up in just a way that the absence of light makes the build appear dangerous. Tight hallways, long, barely-visible corridors and cramped ventilation shafts create a good spot to put your players on the edge. A good environment to perpetuate that atmosphere is one that makes the player feel uncomfortable and awkward, much like an actual dark hallway would.
Lastly, include a forgetton of shortcuts through the build that only the alien can access. The creepiness from the original Alien movie was that you didn't know if the alien was in a room or not. Don't allow the alien to become predictable to the players. The two biggest things about keeping a monster scary is making sure it's appearances are brief, and that in most situations, it's barely visible. Not being able to see the monster makes it that much more threatening.
But that's just my two-cents. I've studied a lot of horror, so if you ever need any ideas or small advice, you can just PM me.
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