Where and when it shows up
Hallway corners, the lobby, and the surgery room — usually once per run. Flickering lights and a distinct sound cue announce its presence before you see it.
The Stalker is the one threat in the roster you're never meant to fight — the entire mechanic is about denying it eye contact, not damaging it. The origin of the Stalker is one of the game's unresolved lore threads: the note found in Ratthew's Room 5 safe ("the shadow now sees everything") is widely read by the community as describing exactly this creature, though that connection isn't officially confirmed.