Attempting...
Aura of Palpable Terror (Su): A darksider exudes can radiate a 20-foot-radius fear aura as a free action. A creature in the area must succeed on a DC x Will save or be affected as though by a fear spell (caster level xth). A creature that successfully saves cannot be affected again by the same darksider's aura for 24 hours. The save DC is Charisma-based.
Creatures within the aura also take xdx points of nonlethal damage each round they remain within the aura. A creature reduced to unconsciousness from nonlethal damage in this manner suffers xdx points of Wisdom drain.
Putting it all together...
Aura of Palpable Terror (Su): A darksider exudes can radiate a 20-foot-radius fear aura as a free action. A creature in the area must succeed on a DC x Will save or be affected as though by a fear spell (caster level xth). A creature that successfully saves cannot be affected again by the same darksider's aura for 24 hours. The save DC is Charisma-based.
Creatures within the aura also take 2d6 points of nonlethal damage each round they remain within the aura. A creature reduced to unconsciousness from nonlethal damage in this manner suffers 1d2 points of Wisdom drain per round they remain within the aura. Victims perceive the damage as coming from an attack (spell or weapon) by the darksider.
Ok, this looks pretty good. What's next, the confusion effect?
Creatures within the aura also take 2d6 points of nonlethal damage each round they remain within the aura. A creature reduced to unconsciousness from nonlethal damage in this manner suffers 1d2 points of Wisdom drain per round they remain within the aura. Victims perceive the damage as coming from an attack (spell or weapon) by the darksider.