Aspect of Tharizdun

So I am working on a monster for a living campaign interactive. This is supposed to be a major climactic challenge for the high level guys of 13th to 15th level. Tell me what you think. I borrowed a lot of ideas from other classes and creatures, but I like the final product as it approximates what I feel an aspect of Big T would be like. Thoughts?

ASPECT OF THARIZDUN CR 20
NE Large outsider (evil, extraplanar)
Init +10; Senses Listen +29, Spot +29
Aura entropic field
Languages Abyssal, Aquan, Auran, Common, Draconic, Ignan, Infernal, Terran
AC 27, touch 15, flat-footed 21
(-1 size, +6 Dex, +4 armor, +8 natural)
Miss Chance 20% (ranged attacks; entropic shield)
hp 180 (20 HD); regeneration 8; DR 15/epic and good
Immune cold, confusion, fear, insanity, mind-affecting, poison
Resist acid 10, electricity 10, evasion, fire 10; SR 29
Fort +15, Ref +20, Will +16
Weakness entropic dependency
Speed 40 ft. (8 squares)
Melee maddening touch +25 touch (2d6+7 plus madness)
Space 10 ft.; Reach 10 ft.
Base Atk +20; Grp +29
Atk Options Combat Reflexes
Special Actions random deflector
Spell-Like Abilities (CL 20th)
At will—confusion (DC 19), darkness, dispel magic, enervation (+25 ranged touch), insanity (DC 22), mage armor, Otiluke’s resilient sphere (DC 19), phantasmal killer (DC 19), shatter (DC 17), touch of idiocy (+25 melee touch), unholy blight (DC 19)
3/day—cone of cold (DC 20), deeper darkness, Otiluke’s freezing sphere (DC 21), polar ray (+25 ranged touch), wall of force
1/day—energy drain (+25 ranged touch; DC 24), greater dispel magic, Otiluke’s telekinetic sphere (DC 23), weird (DC 24)
Already cast
Abilities Str 20, Dex 23, Con 16, Int 25, Wis 19, Cha 20
SQ outsider traits
Feats Ability Focus (maddening touch), Alertness, Combat Reflexes, Improved Initiative, Improved Toughness, Quicken Spell-Like Ability (Otiluke’s resilient sphere), Weapon Focus (maddening touch)
Skills Bluff +28, Climb +28, Concentration +26, Diplomacy +9, Hide +33, Intimidate +30, Knowledge (arcana) +30, Knowledge (history) +30, Knowledge (religion) +30, Knowledge (the planes) +30, Listen +29, Move Silently +37, Search +30, Sense Motive +27, Spellcraft +32, Spot +29
Entropic Dependency (Ex) An aspect of Tharizdun can only exist when it is near an item or location with a strong connection to Tharizdun such as a temple or altar. A holy symbol of Tharizdun is not sufficient. If an aspect of Tharizdun is ever more than 1,000 feet away from such an object, it is immediately banished as per the spell, banishment.
Entropic Field (Su) As a standard action, an aspect of Tharizdun can surround itself with a field of invisible, entropic energy that lasts for 1 minute. The field extends in a 50-foot-radius, centered on the aspect of Tharizdun. All magical healing automatically fails within the entropic field. It takes the aspect of Tharizdun a standard action to dismiss the entropic field.
Once per round, as a free action, the aspect of Tharizdun can force a character within the field (including itself) to reroll an attack, save, or check. The aspect of Tharizdun demands the reroll after it knows whether the attack, save, or check succeeded or failed, but before the exact consequences have been calculated and applied. The result of the reroll takes precedence, even if it’s worse than the original roll.
The entropic field causes wounds created within it to continue bleeding, dealing an additional 1 point of damage per round thereafter (at the end of the aspect of Tharizdun’s turn) as long as they remain with the entropic field. A successful Heal check (DC 15) stops the bleeding. If multiple wounds are suffered, then the creature takes 1 point of damage per wound. The aspect of Tharizdun is subject to the bleeding effect.
Entropic Shield (Su) An aspect of Tharizdun is continually protected by an entropic shield effect, as the spell (caster level 20th). It can be dispelled, but the aspect of Tharizdun can recreate it on its turn as a free action.
Maddening Touch (Su) A creature hit by an aspect of Tharizdun’s maddening touch attack must make a DC 27 Will save or become insane, as per the spell insanity. This is a mind-affecting ability. The save DC is Charisma-based.
Random Deflector (Su) An aspect of Tharizdun has the ability to protect itself from certain attacks with this ability. Using this ability is an immediate action, which means that the aspect of Tharizdun can activate it when it is not its turn. When activated, the random deflector lasts until the beginning of the aspect of Tharizdun’s next turn.
The random deflector redirects ranged attacks, ranged touch attacks, and individually targeted spells (spells designating the aspect of Tharizdun as the sole target, but not spells striking an area in which it happens to be the sole target) so that they instead attack or affect a random target within 50 feet of the aspect of Tharizdun. An aspect of Tharizdun includes itself and its allies among the possible new targets. Any creature targeted by a deflected attack is attacked or affected normally, so an attack roll is made normally against the new target’s AC, the new target receives a saving throw if a spell or effect allows one, and so on. An aspect of Tharizdun can use this ability a number of times per day equal to its Charisma modifier.
Skills An aspect of Tharizdun gets a +8 racial modifier on Hide and Move Silently checks.

Seems to compare somewhat favorably with a Balor, but I can't shake the feeling that this isn't quite CR 20. 180 hit points, even with that DR, strikes me as too low for a creature of that magnitude, and that AC is a joke to any serious fighter at that level. But then again, there is the miss chance, which is as good as non-dispellable, so maybe that can make the difference. The fact that its melee attack is a touch attack is a nice ability though, admittedly.

At 20th level, most parties with time to prepare for the encounter would give everybody Mind Blank as a matter of course, so the Mind-Affecting powers become useless unless the Aspect rolls well on a Dispel check- and it only gets one real chance to dispel, that being with Greater Dispel Magic. Regular Dispel caps at a +10 bonus, which means it needs a 21 on a d20 to dispel a 20th-level caster's effect. And of course, against a 20th-level caster, SR 29 only protects 40% of the time, but then again the Big Bad Balor only has SR 28 so this seems fairly average.

Does the Entropic Dependency mean that if the characters can somehow destroy the thing the Aspect is tied to, then it goes bye-bye even though it technically never moved away from it?

Also, you forgot to note what its regeneration is negated by- if it takes normal damage from Silver or Good weapons, there should be a note to that effect; likewise, if it's a Tarrasque (and nothing can kill it but putting more nonlethal on it than it has hit points and then Wishing it to stay dead) then that should be noted.

And yes, I know this is being designed for characters of much lower level as a BBEG- but if you're going to make it CR 20, don't think of the poor 13th-15th level schlubs who will be facing it- think of the 20th-level characters which, going by base rules setup, should be facing it. So make it CR 20.

Admittedly, 20th level characters should have an easy time with this beast and it should only use up ~25% of their resources to fight it. So only say 20th level characters SHOULD be fighting this thing if you mean to use it as a wandering monster. In that event, I think a group of 20th level characters would probably get what I expect. They would beat it with the loss of a few hp and some spells, but ultimately, it wouldn't be too tough.

Thanks for the points though. The one about regeneration was a good one. I'll fix that.