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.
Parasite Eve and Silent Hill 1. I became a big Silent Hill fan and then eventually came to play and enjoy some of the Resident Evil series.
These games came out in very late teenage-hood for me, but the amount of nights piled around the TV with the bros, pounding beers and bongers, and scaring ourselves…oh man the memories. Those games were absolute rippers, Parasite Eve 2 especially (except for that end boss)