These would be walls that aren't really there. But unless you know they aren't there, for you, they are there. But not really. But they are. But ... well you get the idea.
Anyway, these walls would be marked so they are visible to the player (maybe transparent, or having a bluish glow?), but Beethro would only be able to pass them if he drank an eye-Q potion that let him see through them.
Eyes would be able to penetrate the illusion naturally, as would brained monsters. Every other monster would be barred from them.
(Brained seep would avoid them, unbrained seep would die if they tried to enter them.)
And, thanks to the existence of Master Walls, these new walls would be flawlessly consistent with existing aspects of the Eigth. Other than the horrible horrible pun in the potion name of course.
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"
Now I will repeatedly apply the happy-face rule"
I also forgot the cover sheet.