I came up with this (admittedly light-hearted) monster for a different thread and wanted to know what you thought of it. Your CR system put it at about 24000, but I think that's too high -- what do you think?
(Sorry about the lack of formatting; there's no convenient way to transfer this from Word.)
Tarrasque Swarm
Colossal Magical Beast (Swarm)
Hit Dice: 36000d10+972678 (1,170,678 hp)
Initiative: +15
Speed: 50 ft
Armor Class: 3840 (–8 size, +7 Dex, +3830 natural, +1 haste), touch 10, flat-footed 3833
Base Attack/Grapple: +36000/—
Attack: Swarm (8d8)
Full Attack: Swarm (8d8)
Space/Reach: 1 mile/0 ft
Special Attacks: Distraction, frightful presence
Special Qualities: Carapace, damage reduction 60/—, hive mind, immunity to fire, poison, disease, energy drain, and ability damage, regeneration 5000, resistances, scent, spell resistance 14000, swarm traits
Saves: Fort +18035, Ref +18016, Will +12008
Abilities: Str 85, Dex 25, Con 65, Int 6, Wis 14, Cha 14
Skills: Gather Information +4 (0), Jump +6037 (6000), Knowledge (nature) +599 (1½), Listen +6012 (6000), Search +6000 (6000), Spot +6012 (6000), Survival +6002 (6000) [+600 following tracks]
Feats: Alertness, Awesome Blow, Blind-Fight, Cleave, Combat Reflexes, Deflect Arrows, Dodge, Endurance, Great Cleave, Great Fortitude, Improved Bull Rush, Improved Initiative, Improved Natural Attack (swarm), Improved Overrun, Improved Unarmed Strike, Investigator, Iron Will, Lightning Reflexes, Mobility, Power Attack, Run, Toughness (6)
Epic Feats: Armor Skin (1900), Blinding Speed (2880), Damage Reduction (20), Dire Charge, Energy Resistance (acid) (15), Energy Resistance (cold) (15), Energy Resistance (electricity) (15), Energy Resistance (sonic) (15), Epic Endurance, Epic Fortitude, Epic Reflexes, Epic Speed, Epic Toughness (33), Epic Will, Great Constitution (30), Great Dexterity (9), Great Strength (40), Improved Spell Resistance (6984), Penetrate Damage Reduction (adamantine, cold iron, mithral, silver, and any 6), Reflect Arrows, Superior Initiative
Environment: Any
Organization: Solitary
Challenge Rating: ~24000
Treasure: None
Alignment: Always neutral
Advancement: —
Level Adjustment: —
Distraction (Ex): Any living creature that begins its turn with a swarm in its space must succeed on a DC 18037 Fortitude save or be nauseated for 1 round. The save DC is Constitution-based.
Frightful Presence (Su): The tarrasque swarm can inspire terror by charging or attacking. Affected creatures must succeed on a DC 18012 Will save or become shaken, remaining in that condition as long as they remain with 60 feet of the tarrasque swarm. The save DC is Charisma-based.
Carapace (Ex): The tarrasque swarm's armorlike carapace is exceptionally tough and highly reflective, deflecting all rays, lines, cones, and even magic missile spells. There is a 30% chance of reflecting any such effect back at the caster; otherwise, it is merely negated. Check for reflection before rolling to overcome the creature’s spell resistance.
Hive Mind (Ex): Any tarrasque swarm with at least 1 hit point per Hit Die (or 36000 hit points, for a standard tarrasque swarm) forms a hive mind, giving it an Intelligence of 6. When a tarrasque swarm is reduced below this hit point threshold, its Intelligence drops to 3.
Regeneration (Ex): No form of attack deals lethal damage to the tarrasque swarm. The tarrasque swarm regenerates even if it fails a saving throw against a death effect. If the tarrasque swarm fails its save against a spell or effect that would kill it instantly, the spell or effect instead deals nonlethal damage equal to the creature’s full normal hit points +10 (or 1,170,688 hit points). The swarm is immune to effects that produce incurable or bleeding wounds, such as mummy rot, a sword with the wounding special ability, or a clay golem’s cursed wound ability.
The tarrasque swarm can be slain only by raising its nonlethal damage total to its full normal hit points +10 (or 1,170,688 hit points) and using a wish or miracle spell to keep it dead.
Resistances: A tarrasque swarm’s epic feats give it resistance 150 to acid, cold, electricity, and sonic.
Skills: A tarrasque swarm has a +8 racial bonus on Listen and Spot checks.