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.
What do you mean by formative horror? A horror game that had ultimately planted your interest in the horror genre?
That, or really anything that you were exposed to at an early enough age to influence your tastes or how you contextualized the themes it explored later in your life.