In Amnesia, the devs did with the Shadow's macabre growth; slowly reaching the player as the player runs towards something I cannot remember after escaping a prison, I should replay the game sometime.
You can check the original levels for that script, there's that one level I noted above which has a creeping fog as a wall; but I highly doubt the damage-zone is parented to anything, rather than that the damage-zone is part of the growing influence of the shadow on the floor; I.E: Many, many separate different tiles of zones....Well, Amnesia is already annoying to script in; more annoyance is not that bad.
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By "Generic walls" I meant that they lack any color, the walls itself already look esoteric; to leave it 'alone' is already out of place. I suggested adding more stuff onto it to make it seem more rich, although I think an outer wall
should generally be made out of stone, but yours specifically needs to be taller to actually hide the castle behind it.
So, the decals will not totally 'fix' the wall; if it works well it can be enhanced. Try to take some reference to see what is that exactly you're missing, or how important at all you think that aesthetic is to you. (If that's the last time we see that wall, I may understand why you will be reluctant to spend more time on it. But if the innards of the castle are engulfed with that same wall everywhere...It's better to rethink it)
Again, go look at references if you want to do anything with it, if you don't : Just fixate yourself on continuing the story, return to fixing that level when you finished more than a single level.