ليكون المعنى: شر عظيم فظيع
So that the sense becomes: a terrible, overwhelming evil.
The indefinite «شر» need not merely indicate an unknown instance. In this context it can express magnification and alarm, intensifying the magnitude of the harm.
Thus indefiniteness and restriction perform different tasks. Leaving a noun unspecified may serve to heighten its character rather than conceal factual identity.
This effect is context-dependent; an indefinite noun is not intrinsically an expression of praise or dread.