For the Cleric that wants to advertise... [Feat]

[Lysander]

Holy Aura
Prerequisite: Level 10 in a class that uses divine spells and requires a good alignment

You radiate holy energy and are surrounding by a faint visible field of golden light. The effect is more potent than a simple illusion and can be felt emotionally as well as seen. Most good or neutral NPCs will be awe struck and treat the character with reverence, granting a +2 diplomacy bonus on checks related to ethical arguments or pleas to one's better nature. The light casts as much illumination as a single candle, and when active causes a -2 penalty to hide checks and a -10 penalty to disguises (unless trying to pose as someone that glows). The Holy Aura can be supressed at will.

[de-trick]

so basically a holy aura power up but if its a 10+ level feat id say its should make undead and evil outsiders weaker like a -2 to all rolls

[Poppatomus]

Isn't there already a feat similar to this in the BoED?

[Triaxx]

Would be very interesting for impersonating a divine messenger, or Solar. Or a glowing bear for a druid.

[Hazkali]

Isn't there already a feat similar to this in the BoED?

Yes, it's called Nimbus of Light and doesn't require your character to be 10th level, although it is an [Exalted] feat so it has the uber-good prerequisites.

Nimbus of Light is also the prerequisite for a number of cool (if not particularly powerful) exalted feats such as Stigmata and one other whose name eludes me that increases the range and potency of the Nimbus, which is basically the feat that De-Trick is suggesting.