It’s basically a short story told in the second person. There will typically be some sort of chart for details about performers, effects, etc., but it starts with, “You approach a dilapidated building.”
My treatments are written in the “HHN” style. During the rehearsals for Screamhouse, they read us the treatment so we understood our role in the context of the story. Because that’s what haunted houses are—stories that guests participate in.
And all the treatments I’ve seen from various haunts are based in that sort of style.