That was easy!
Here's a start at a modification, but we need to decide how they wake up. Recovering 1 hp/HD/hour means they're going to stay knocked out for a while. It would be better for them to wake up faster; maybe have them wake up after 1 round with all nonlethal damage healed?
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. Victims perceive the damage as coming from an attack (spell or weapon) by the darksider.
I like that (with a few edits)! First, it should be Sp if SR applies, so it has a caster level. But the great thing about it is that it seems to include everything we wanted all in one ability.
Here's my modification, so see how you like it.
Nightmare Projection (Sp): A darksider can trap opponents in a waking nightmare, affecting X intelligent creatures within 60 ft. There is no saving throw, but spell resistance applies and nightmare projection does not affect creatures with immunity to fear or mind-affecting powers. Creatures trapped within the nightmare projection can act normally, but perceive their surrounding as whatever the darksider desires them to -- an ally may appear a terrible monster, or a charnel chasm may become a perfumed garden.
A darksider can create phantasmal monsters or spells to attack its nightmare victims. These do between 1 and 4d6 points of nonlethal damage (half with a DC Z Will save). The darksider can attack its victims through a nightmare projection in any combination it desires. Note that a nightmare projection can also disguise the source of lethal damage -- for example, two victims may unknowingly attack each other because the nightmare projection made them see each other as monsters.
If the victim of a nightmare projection falls unconscious, the victim can escape the nightmare if they succeed at a DC X Will save. If this save succeeds, the victim becomes immune to the nightmare projection of that darksider for 24 hours and wakes with all nonlethal damage healed after 1d8? rounds. Should the victim fail the Will save, they take 1d3? Wisdom drain and believe they have been killed, falling unconscious for 1d8? rounds and then waking without any illusory damage (the victim believes the darksider has resurrected them in order to continue tormenting them). The darksider gains 5 temporary hit points every time it drains a victim's Wisdom. A victim whose Wisdom reaches 0 due to a nightmare projection dies.
This ability is equivalent to a X-level spell. Caster level X.
Could probably still use some work, but it looks good. Maybe we should specify that, no matter how many times it uses its nightmare projection, it can affect only a max of X creatures at a time or something.
You don't have to worry about all that with most SLAs; it's just standard business.![]()
I slightly favor Su.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.