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.
My childhood originally consisted of the SNES and N64, but I mostly remember things from the N64. I don’t recall a lot of true horror games for the platform. There was Resident Evil 2, but the scariest things I remember were specific areas of Ocarina of Time such as the shadow temple or beneath the well.