D&Dtools1

The normally formidable aura that marks you as a servant of a higher power recedes, leaving few traces of your allegiance visible to creatures that can see such marks.
This spell masks the target's aura, making it appear less powerful than it really is.
For every two caster levels, the target of a subdue aura spell appears to have 1 fewer Hit Die than it really does.
If the target has no aura, the spell has no effect.