Also how does Reversal treat subdual damage?
Edit: Nevermind that question, it doesn't matter.
I have an idea on Reversal for you.
For the Class feature:
Reversal (Ex): Nilbogs suffer from an odd condition they refer to as 'the reversal' which can grow to encompass more effects as the Niblog becomes more powerful. The effects of positive and negative energy are reversed for the Nilbog. This means that positive energy effects (such as Healing spells) deal damage equal to the amount they would normally heal, and that negative energy effects (such as Inflict spells) heal damage equal to the amount they would normally cause. A nilbog gains any extra hit points above its normal total as temporary hit points, though it can never have more than twice its normal hit points. (Excess hit points are simply lost.). The effects of energy drain/negative levels caused by negative energy are also reversed, gaining bonuses instead of penalties (a Nilbog cannot gain spell slots due to this). If the Nilbog is targeted by a positive energy effect that explicitly removes a specific number of negative levels, the Nilbog instead suffers that number of negative levels.
With this change, reduce the Nilbog level adjustment to +1. I also like the explicit mentioning of what happens with negative levels since they're negative energy effects.
Then add a feat. Something like:
Damage Reversal
Your reversal has new and odd effects.
Prerequisites: Nilbog, 4 or more Nilbog Racial Feats
Benefits: When struck by any attack or effect that would normally damage the nilbog, it actually gains hit points equal to the damage the attack or effect would have otherwise dealt. A nilbog gains any extra hit points above its normal total as temporary hit points, though it can never have more than twice its normal hit points. (Excess hit points are simply lost.). The nilbog can only be damaged through the use of curative magic and effects (cure light wounds and healing potions, for instance). Curative magic deals damage equal to the amount it would normally heal.
