For me it was Dead Space 2 when I was 12-ish. For reference, at this point the most gruesome/gorey/violent media I was exposed to was the Halo 3 Flood levels and the original 2 Alien movies.
I had way too much fun playing it to be traumatized by it at the time, although when i was old enough to understand the horror of the whole "your memories and experience becoming food for a god-like being that has absolutely zero respect for your existance", that did inform my perspective of other media such as Evangelion or Childhood's end when I watched them for the 1st time.
What was your equivalent to this? I've heard the Resident Evil games are quite common for this but I want to hear your perspective.
Shalebridge Cradle is one of the most terrifying fucking levels I’ve ever played.
Like, you know something bad and creepy is coming up before the level. The dialogue hints, the general unease around the building, random things here and there in the game, etc.
Yet it still hits you like a brick to the face. Nothing prepared me for it.
No surprise the lead designer for that level went on to design one of the more creepy parts of BioShock as well.