Rimworld

Marked as a spoiler because its a monster from Anomaly. The thing is, these things are not scary, because they don't have the AI necessary to capitalize on their invisibility. They act like typical raiders, meaning you can place your tough melee guys in a chokepoint, and they'll come to get their skulls bashed in.

It would probably be better if they instead acted like predatory animals, milling around on the map, and occasionally hunting one of your colonists. If they'd then avoid groups of colonists, while also always attacking in a group themselves, they'd be a truly terrifying monster. Basically, you'd have to hide out in your base, or go out to hunt them. And if you do choose to wait them out, there would be no indication that they've left.