Clive Barker's Undying carries the distinction as one of the first games to carve an expression of terrified horror in my face from start to end. I swore I'd never revisit it, but it's on GOG now for $6, an absolute steal for one of the best and truly frightful first-person survival horror games out there.
Sure, it's not as weird as previous new(old?)comer Omikron , but Undying still keeps its magic of piling on creepy mysticism, nightmarish enemies, and lots of frantic shooting. All because the Covenant siblings couldn't resist blurting out a profane ritual. Silly Covenants.
As with the rest of GOG's library, Undying's olden 2001 graphics are kept pristine while taking a bump in compatibility for Windows XP, 7, and 8. For more nostalgic incoherent screaming, here's the intro trailer.