From: System Shock 2 (PC, 1999)

Before GlaDOS broke our hearts in Portal, there was SHODAN, a rogue AI who all but defined System Shock 2, the cult-classic predecessor to Bioshock. And while some of the monsters on this list inspired horrible dread with the suspicion that they were always watching, SHODAN was always watching - not that you necessarily knew it at the time.

In fact, for the first part of the game, you didn't even know SHODAN was involved. Instead, you were guided from afar by Dr. Polito, a voice whom you came to trust… until you found out she'd been dead the whole time. In reality, your actions had been guided and manipulated by SHODAN, who was using you to eliminate the rogue army of horrors that she'd created from the bodies of the ship's crew.

That's... not so bad, right? Well, we may have failed to mention that SHODAN is completely insane, a rogue AI who believes herself to be divine and sees you as nothing more than an insect to be used and then crushed. And she's in complete control of the ship. Once she reveals herself, all bets are off, and the realization that you're alone against her and her creations is a scare that stays with you through the remainder of the game. And SHODAN will constantly remind you of it as she monitors your every move, taunting you mercilessly as her warped creations shamble after you. Creepy.

 
From: Silent Hill 2 (PS2, Xbox, PC, 2001)

Fan-favorite Pyramid Head is possibly the most horrifying character ever to have a cult following among gamers. A pale-skinned, zombie-like torturer in a leather apron and creepy helmet, Pyramid Head (or Heads, once you find out there's more than one) slowly stalks you throughout Silent Hill 2. Sometimes he'll attack you directly, while other times, you'll narrowly avoid running into him as he torments (many would say "rapes") some of the town's other monsters. Sometimes he'll just stand at the end of a hallway, staring impassively at you from under his impenetrable visor.

Whatever the case, he's an incredibly dangerous creep who carries a huge knife, and he was one of the biggest reasons why Silent Hill 2 remains one of the best horror games ever made. He's so iconic, in fact, that the Silent Hill movie just wouldn't have felt complete without his mute, eight-foot-tall presence.

But there's more to Pyramid Head than just a helmet and a murderous disposition. When SH2's protagonist James Sunderland arrives in Silent Hill, the town creates Pyramid Head - along with a slew of other monsters - as a means of tormenting James for his sins. Weirder still, Pyramid Head is said to be a direct projection of James' own feelings of guilt and self-hatred, meaning that it's entirely likely that the face under that helmet is James' own. We may never know for sure, although Pyramid's brief appearance in a Silent Hill: Homecoming trailer seems to hint that there might be more answers down the road.

All metaphysical eeriness aside, Pyramid Head is arguably the most badass character on this list, mainly because he's indestructible. Not nigh-indestructible, like the Berserker, or constantly regenerating like the Nemesis - we mean your bullets don't actually seem to hurt him. He isn't defeated at the end of boss battles, he gets bored and leaves. And when he's finally killed - at the end of a fight with two of him - he actually commits suicide, indicating that he's so disgusted by your puny efforts that he'd rather just end it all than continue dealing with you.

Or maybe it's supposed to be symbolic of James' final defeat over his own self-loathing or blah, blah, blah. Whatever. Pyramid Head's freaky as hell, all right?