What do you mean by formative horror? A horror game that had ultimately planted your interest in the horror genre?
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I had played other horror games before it but the first one I became obsessed with was Resident Evil 4. I think I mostly just enjoy survival horror type games for the challenge, because other horror games have never really held much of an interest to me unless they have some kind of survival aspect.
Phantasmagoria by Sierra Online.
One of two. Ghosts and Ghouls on snes, I was like 4 but wasn't scared. And the other one is not a horror game but for 6 year old me it was scary as shit, Halo CE. The Library level and the one before, 343 Guilty Spark, were hell for me. Beating those levels was a goddamn relief.
Shivers - that game absolutely nailed the atmosphere, and for the players that don't know the ixupi hotspots and the game's tricks, it's genuinely terrifying. I'd say give it a look, on GoG you could do worse for six bucks.
Not sure it counts, but I played a loooot of Killing Floor in my early teens.
Fatal Frame II: Crimson Butterfly
The old Japanese village scene with the super dark graveyard area. Somehow the image still stuck hard in my head.
Master of Darkness on the Sega Master System. The soundtrack was haunting.
Thief, the dark project + my brand new EAX 4 speaker surround system... hoooly shit
Eyes