The nightmare resembles more a self-defense mechanism and a cleansing system of this world, specifically designed to eliminate those unstable elements.
Herag himself hails from another plane, thus upon becoming an official exorcist, he triggered this nightmare mechanism.
Luckily, his own strength was already formidable, so he wasn't particularly afraid of that nightmare.
Even without the existence of the black membrane, Herag could handle it, though it would cost him more effort.
However, if Herag hadn't possessed his prior strength and only relied on the power gained through demon aura, defeating the nightmare would be nearly impossible.
The black membrane can dissolve the ray, but the ray is the nightmare's final resort.
Without his prior strength, Herag would find it difficult to force the nightmare into using its ray, not even reaching that stage.
