Pshaw!
I was looking over the yuan-ti anathema, and it has this ability:
Mass Aversion (Sp): An anathema can psionically create a compulsion effect targeting all enemies within 30 feet. The targets must succeed on a Will save (DC 27) or gain an aversion to snakes for 10 minutes. Affected subjects must stay at least 20 feet from any snake, yuan-ti, or ti-khana creature, whether alive or dead; if already within 20 feet, they move away. A subject can overcome the compulsion by succeeding on another Will save (DC 27), but still suffers from deep anxiety. This causes a -4 reduction to Dexterity until the effect wears off or the subject is no longer within 20 feet of a snake, yuan-ti, or ti-khana creature. This ability is otherwise similar to antipathy as cast by a 16th-level sorcerer.
This actually seems more powerful than the form of madness you indicated above. Maybe we can combine the two, and drop the insanity aura?
Other yuan-ti abilities to consider:
Alternate form (Sp): All yuan-ti can assume the form of a Tiny to Large viper (see the Snake entry, page 280) as a psionic ability. The yuan-ti loses its natural weapons (if any) and gains the natural weapon of the viper form it assumes. If the yuan-ti has a poisonous bite of its own, it uses its own or the viper's poison, whichever is more potent.
Chameleon Power (Sp): An abomination or anathema can psionically change the coloration of itself and its equipment to match its surroundings, gaining a +10 circumstance bonus on Hide checks.
Detect Poison (Sp): All yuan-ti have the psionic ability to detect poison as the spell (caster level varies).
Produce Acid (Sp): An abomination has the psionic ability to exude acid from its body, dealing 3d6 points of acid damage to the next creature it touches, including a creature hit by its bite attack. If the yuan-ti is grappling or pinning a foe when it uses this power, its grasp deals 5d6 points of acid damage. The acid becomes inert when it leaves the yuan-ti's body, and the yuan-ti is immune to its effects.
or
Dissolving Touch (Sp): An anathema can psionically exude acid from its body, dealing 7d6 points of acid damage to anything it touches. If it uses this power when grappling or constricting an opponent, the acid deals 10d6 points of damage instead. The acid is effective only on the round when this power is used, and it becomes inert when it leaves the anathema's body.
Per the demon lord template:
Spell-Like Abilities: All demon lords have the following spell-like abilities in addition to any the base creature possesses:
Spell-Like Abilities:
At will—astral projection, blasphemy (or word of chaos for obyriths), desecrate, detect good, detect law, greater dispel magic, greater teleport, plane shift, shapechange, telekinesis, unhallow, unholy aura (or cloak of chaos for obyriths), unholy blight (or chaos hammer for obyriths);
3/day—any one symbol.
Each demon lord also possess two additional at-will spell-like abilities, two additional 3/day abilities, and two additional 1/day abilities. These additional spell-like abilities should be chosen to match the demon lord's "theme".
A demon lord's base caster level for spell-like abilities ranges from CL 20th to 25th, based on the demon lord's overall CR (starting at CL 20th and adding +1 to its CL for every 3 points its CR exceeds 18, to a maximum of CL 25th at CR 33).
Anathema Spell-Like Abilities: 3/day -- animal trance (DC 18), baleful polymorph (DC 21), cause fear (DC 17), deeper darkness, entangle (DC 17), haste, neutralize poison, suggestion (DC 19), unholy blight (DC 20); 1/day -- blasphemy (DC 23), unholy aura (DC 24). Caster level 20th. The save DCs are Charisma-based.
Abomination Anathema Spell-Like Abilities: At will--animal trance (DC 16), entangle (DC 15); 3/day--deeper darkness, neutralize poison (DC 18), suggestion (DC 17); 1/day--baleful polymorph (DC 19; into snake form only), fear (DC 18). Caster level 10th. The save DCs are Charisma-based.