The creature touched may make a Will save to negate the spell. Otherwise, you cause the creature (hereafter referred to as "the target") to emanate magical vibrations that cause others to view him with suspicion. Others who find themselves inside the area of effect must make a Will save (or, rather, the DM secretly rolls it for them). If they do, they still view the target suspiciously, but those who fail are outright hostile. Any statement the target makes that's not an obvious fact will be met with disbelief. Saying "the sun is bright" doesn't cause much of a stir, but a statement like "a wizard just cast some sort of spell on me" or "there are orcs hiding just behind that hill" will be utterly disbelieved.
Creatures with true sight or similar magics can detect the aura, but are still affected as if they made their save. This spell ceases to function while the target is inside a zone of truth or similar effect, but turns back on as soon as the target leaves the area. Spell resistance works to protect both the target and anyone inside the area of effect.
Note that the language-dependant and mind-affecting properties of this spell only come into play with those caught in the area of effect, and not the target.
Unbelievable Aura counters and dispels Credible Aura.
Material component: Five or more hairs from a wolf that have been soaked in a young boy's tears.