Hmm, you may be stuck trying to advance some critters then.
Fiendish Codex has the demon prince Dagon at about CR 21 or so, iirc... but then, you have the evil aspect goin' on.
Hey, wait- here's a nasty, nasty creation I used on my epic party a while back- you might actually have to tone it down...
The Ur-Fish (CR 22): Gargantuan magical beast; HD 30d10+360; hp 525; Init -1; Spd Swim 50’; AC 36/50% (-4 size, -1 dex, +25 natural, +6 expertise), touch 11/50%, flat-footed 26/50%; BAB/Grapple +30/+61; FA Bite +23 melee (6d6+42/19-20) (includes adjustments for Combat Expertise and Power Attack); Space/Reach 20’/15’; SA Frightful presence, spell-like abilities, sweeping bite; SQ DR 10/epic, darkvision 120’, fast healing 10, immunity to negative energy, petrification, poison, positive energy and radiation, inky corona, resistance to electricity 15 and sonic 15, SR 32; SV Fort +29, Ref +16, Will +15; Str 40, Dex 8, Con 34, Int 17, Wis 16, Cha 13; Align NE.
Skills & Feats: Bluff +29, Concentration +21, Diplomacy +24, Listen +36, Search +16, Sense Motive +28, Spot +36; Cleave, Combat Expertise, Epic Reflexes, Great Constitution, Greater Weapon Focus (bite), Improved Bull Rush, Improved Critical (bite), Improved Natural Attack (bite), Iron Will, Power Attack, Weapon Focus (bite).
Frightful Presence (Su): Any creature within 100’ of the ur-fish that sees it attack, use its sweeping bite or charge must make a Will save, DC 26, or become shaken for as long as they can see the ur-fish and for one hour afterward. Regardless of whether it succeeds or fails, once a creature rolls this save they are immune to the ur-fish’s frightful presence for 24 hours. The save DC is charisma-based.
Spell-Like Abilities (Sp): At will- see invisibility, shatter; 1/day- bestow curse, blindness/deafness, darkness, greater dispel magic. Caster level 40th; save DCs 11 + spell level. The save DCs are charisma-based.
Sweeping Bite (Ex): As a standard action the ur-fish can bite all targets in a 10’ cube, rolling a single bite attack against all of them. Any creature that suffers damage must make an opposed grapple check or be swept into the ur-fish’s mouth. On its next turn, as an attack action, the ur-fish can attempt another opposed grapple check to swallow its victims whole. Swallowed victims suffer 2d6+15 points of bludgeoning damage plus 1d6 points of acid damage for each round within the monster’s belly. The belly is AC 30, and each victim must either succeed at two opposed grapple checks or else inflict 60 points of damage to the stomach with a slashing weapon in order to free themselves. Because of the immense size of the ur-fish’s gullet, characters swallowed can wield any weapon and can take any action. Unfortunately, within the creature’s gut they are constantly being thrown around by stomach motion, and are effectively staggered. Moreover, victims must make Concentration checks to cast a spell or manifest a power (DC 10 + last damage dealt).
Inky Corona (Su): The ur-fish is always surrounded by a corona of ink that gives it concealment (50% miss chance). The inky corona cannot be eliminated without a wish, limited wish, alter reality or miracle.
**Note that the ur-fish typically attacks with a -6 from Improved Combat Expertise and a -10 from Power Attack.
Here's the flava I used in my notes:
Here, lurking in contemplation, is a powerful and unique creature that began life as a simple fish. But then it became a kuo-toa wizard’s familiar; and then, both of them were exposed to strange Underdark radiations and planar energies; and to top it all off, the radiation from the reactor here have changed this thing into a terrible intellect and a magical entity of incredible malignancy. It is constantly surrounded by an inky corona, but it looks like some mad Jack Kirby fish-demon. The Ur-Fish speaks Kuo-toan, Undercommon, Drow and Dwarven. It is almost three centuries old and very potent in its power; it has never met anything else of comparable strength, and so it is very arrogant.