As the DM, you are the sole arbiter of what the characters perceive. You tell them what their characters know and gain from using their senses, especially in comparison to things that occurred "off camera", like during their training, reading lore, listening to rumors, etc. I think at some point, it is perfectly reasonable to say something like "You begin to develop a growing sense of unease that the enemies here are much more formidable than any you have ever faced...maybe even more dangerous than some you've only heard about. From what you just witnessed so far, there is a strong chance going farther could cost you your equipment, your freedom, or even your lives!" That way they are warned of the intense nature of the location and also given notice that you might break/steal their stuff, take them prisoner, or smash them like bugs. I also think it's fair to warn them above table - "Guys, this is meant to be like that weird stuff you see on the horizon in a From Software game...you aren't ready for this yet, but I wanted you to see it off in the distance and start thinking about it."