What makes for a compelling horror game? Is it gore? Suspense? Being doggedly chased and outmatched by an opponent who can literally punch through solid concrete? Why, all of the above, of course!
There's a reason why the same checklist of horror motifs crops up in all corners of the genre. Simply put: they're deadly effective at making the player feel helpless but also resolute to beat the odds and fight another day. And nothing else promotes that ideology more than a villain that can't by killed via normal means, but blatantly breaks the internal rules of what a video game should allow.
Mr X cares not for your preconceived notions of what a save room should (and shouldn't) allow to enter, just as Slender Man refuses to obey the laws of physics. They play by their own rules, and it just so happens that the most important of all is killing every living being in sight. Including you.
Welcome to the house of horror.
10. Everything - Amnesia: The Dark Descent
Why no other aspiring contender for the horror crown hasn't recycled Amnesia's fascinating sanity (or lack thereof) mechanic truly boggles the mind.
Frictional Games' first mainstream hit, despite being a decade old, remains in the modern consciousness, not because of its grotesque monster designs (though they're pretty grim, to be fair), but for a semi-realistic depiction of what it would be like to slowly lose one's mind in a creepy as all hell castle.
Now, don't get me wrong, I hid in a wardrobe and wailed like a banshee whenever Daniel caught even the slightest glimpse of another living being, but it's the darkness - not what lies within it - and its all-encompassing nature that serves as the real 'villain' of The Dark Descent. As humans, we're hardwired to be wary and defensive (doesn't count if you're inebriated, obviously) in the after-hours portion of any given day; imagine having to be that tightly-wound 24/7.
It's like an all-encompassing Jack-in-the-box that just keeps winding away until finally, the tension bubbles over and births your worst nightmare.