Cleon
Legend
Nonmagical barriers do not obstruct an Unhallowed Aura. A hallow spell will block the effects of the aura, while a protection from evil, or magic circle against evil spell will weaken the Aura by 2 (e.g. a +4 turn resistance becomes +2 turn resistance).
Incidentally, my original draft used a "level counterbalance" mechanism for apotropaic spells, but I dropped it as too complicated.
e.g.:
A hallow or magic circle against evil spell will weaken an Unhallowed Aura on a level-by-level basic. For example, a 9th level hallow will weaken a 13th level Unhallowed Aura to 4th level, resulting in a -2 turn undead penalty. This will partially counter the +4 sacred bonus to turn undead checks of the hallow spell, for a net +2 modifier.