The Hidden is one of my favourite games: it's a mod for Half-Life 2 Deathmatch in which one side is a SWAT team, and the other side is a slippery, invisible, singular monster (now I've mentioned it, I am doomed to play it all weekend). The reason it's in my head is because of Damned, another asymmetrical multiplayer game that popped up in my inbox (thanks, Daniel). But where The Hidden was focussed on action, Damned looks like a multiplayer Amnesia.
Five players are locked in an abandoned hotel. Four play as the survivors, hunting for the way out. Doors are locked, drawers are filled with helpful keys, and you have no weapons and no way of fighting off the monster if it decides it wants to eat you. All you have is a flashlight. The fifth player is the monster.
The monster has two states: as a ghost, and as a monster. As a ghost, you'll be unable to see the players, though you'll be able to haunt the place. The physical state will allow you track footsteps and attack the survivors.
I don't find horror games particularly scary: when something is programmed to leap out at me, I've always felt it was cheap contrivance. And so many games telegraph their scares (I'm looking at you, Dead Space), that even the element of surprise is lost. It's mostly just a thing that I guessed was coming. But in Damned if things are happening then it's because whomever is in control of the monster is right there in the room, fucking with you. That's a really smart idea that works well in The Hidden.
I hope the players have some way of escaping a monster when he's on their trail, because being marked for death doesn't sound like too much fun, and I'm not sure how being blind to the player movements will enable you to effectively haunt them. Maybe you can see their flashlights? But there's a core of an idea here that I really like. Five friends together on a server could have a lot of fun with this, as long as there's some semblance of balance.