I developed these stats not to propose a combat threat to players, but rather to establish exactly how powerful a threat to A'Tuin must be for plots that threaten the insanely large turtle. Comparatively, spells and threats to destroy any other planet would need to be of a similar stature, so you can use these numbers as a starting point. If you want to make the epic spell "Death Star Super-Laser", it's going to need to deal well more than eleven billion points of damage. So take note.
Size rules from the Immortal's Handbook.
The Great A'Tuin CR 84,431,558
Female Star Turtle (Animalized Double-Macrobe Dragon Turtle)
True Neutral Mega-Gargantuan Animal (Aquatic)
, Senses ; ; Scent, Low-light vision
AC 137,469,003, touch -4,194,287, flat-footed 137,468,996 (-4,194,304 size, +7 dex, +141,663,300 natural)
hp 10,969,646,080 (201,326,592d8 + 10,063,676,416 HD); DR 15,234,372/—
Resist cold, fire, and electricity 5000; radiation 6000
Spd fly 152 miles (clumsy) (802,560 feet, 160,512 squares)
Melee Bite +130,023,532 melee (1920d10 + 108), 2 flippers +130,023,528 melee (960d10 + 108)
Base Atk +134,217,728;
Space 10,000 miles ft.; Reach 6,666 miles
Atk Options Swallow Whole, Snatch, Capsize
Combat Gear Sun, Moon
Abilities, , , , ,
Feats Awesome Blow, Blinding Speed x14400, Blind-Fight, Combat Reflexes, Cleave, Damage Reduction x5,078,124, Deflect Arrows, Energy Resistance (Cold) x500, Energy Resistance (Fire) x500, Energy Resistance (Electricity) x500, Epic Speed x8,800, Epic Toughness x20m, Exceptional Deflection, Extended Life Span x70, Fast Healing x1m, Great Cleave, Great Dexterity x15, Great Fortitude, Great Wisdom x9, Improved Bull Rush, Improved Combat Reflexes, Improved Natural Armor x41m, Improved Unarmed Attack, Infinite Deflection, Perfect Health, Power Attack, Radiation Resistance* x6000, Reflect Arrows, Snatch
Skills Hide -92 (-84 when submerged in nebula), Listen +1, Spot +1, Survival +2, Swim +116
Possessions The Discworld, The Discworld's Sun, The Discworld's Moon
Environment Deep space
The interesting thing about the human condition is that we think that big things are amazing. But honestly, the fact that there exists a turtle the size of a planet isn't nearly as amazing as the fact that turtles exist at all.
Great A'Tuin is a character created by Terry Pratchett as part of his Discworld book series. Great A'Tuin the Star Turtle exists because every probability curve needs a "far end", that and the fact that some creators (meta-deities, if you will) enjoy a joke. And besides, it's just right that there be somewhere in existence where things people believe are true, and some people believe that the world is not only flat, but carried on the back of a gigantic turtle. To be honest, some people also believe that the world is instead carried on the back of four gigantic elephants, but here we win out again. Standing on A'Tuin's back are four gigantic elephants who carry the Discworld (and are named Berilia, Tubul, Great T'Phon, and Jerakeen).
Included with A'Tuin and the elephants and the great big disc are a couple of small moons and a tiny sun, all of which orbit the disc by traveling between the elephants' legs.
The inhabitants of the Discworld are unaware of A'Tuin's sex, which might be a critical peice of information if they should ever encounter a second star turtle. Will A'Tuin fight the new turtle for territory or try to mate with it? An entire world might hang on the balance of this question. Other discworld scientists are more interested in answering simpler questions, such as measuring the exact albedo of A'Tuin's left eye (visible from the rim).
Star Turtles are peaceful creatures who carry worlds on their backs. A star turtle will avoid danger when possible, and fight only when unavoidable. It will snake out its neck to snatch at passing meteors or comets that might threaten the world on its back.
Capsize (Ex): A turtle that surfaces under a ship less than 4,000 miles long "capsizes" the vessel 95% of the time. It has a 50% chance to capsize a vessel from 4,000 to 10,000 miles long and a 20% chance to capsize one over 10,000 miles long.
Snatch (Ex): The turtle can choose to start a grapple when it hits with a claw or bite attack, as though it had the improved grab special attack. If the turtle gets a hold on a creature three or more sizes smaller, it squeezes each round for automatic bite damage.
The creature can drop a creature or object it has snatched as a free action or use a standard action to fling it aside. A flung creature or object travels 1d6 x 1000 miles, and takes 1d6 points of bludgeoning damage per 10 feet traveled assuming it hits something. (1000 miles = 5,280,000 feet = 5,280,000d6, average 1,848,000 damage) DMs may wish to convert up to all but 20d6 of this damage to fire damage if the object hit has an atmosphere.
Swallow Whole (Ex): A'Tuin can try to swallow a grabbed opponent of Mega-Large or smaller size by making a successful grapple check. Once inside, the opponent takes 1280d10+108 points of crushing damage plus 1280d10+108 points of acid damage per round from the turtle’s digestive juices. A swallowed creature can cut its way out by dealing 650 million points of damage to the turtle’s digestive tract (AC 118,052,750). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.
The turtle’s gullet can hold 2 Mega-Large, 8 Mega-Medium, 32 Mega-Small, 128 Mega-Tiny, 512 Mega-Fine, or (for ease of record keeping) any number of smaller creatures.
Improved Life Span (Ex): Unlike normal turtles, who live to an average age of 100, star turtles live far longer, averaging 212 trillion years old at the time of death by old age. (this ability is granted via 70 applications of the Extend Life Span epic feat)
Skills: A turtle has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line. While a Star Turtle has a listed "fly" speed, you may treat flight through space like swimming.
New feat: Radiation Resistance [EPIC]
Prerequisite: Constitution 30
Benefit: You take one less point of strength, dexterity, and constitution damage from radiation poisoning.
Special: You may take this feat more than once, the effects stack.
New Artifact: The Discworld's Sun
A gigantic nuclear furnace about 1 mile in diameter, the Discworld's sun is considerably smaller than "Roundworld"'s sun, but is just as hot. It orbits A'Tuin slightly further away than the moon, which is of comparable size. Creatures and objects touching the sun are dealt 16,666d6 fire damage per round. The sun is of a liquid consistency, and creatures that can survive the temperatures (few exist) can swim in it. Being completely immersed in the sun's fire deals 166,666d6 fire damage per round. Creatures which travel to the absolute center of the sun are dealt 1,666,666d6 fire damage per round.
Fire Immune creatures are not immune to the fire of the sun. Creatures possessing Fire Immunity instead gain Fire Resistance 50 (which stacks with any other source of Fire Resistance). Creatures possessing the Fire subtype gain an additional Fire Resistance 50 (stacking with the resistance gained by fire immunity, above), and creatures made primarily out of fire (such as fire elementals) gain an additional Fire Resistance 50.
Creatures which normally do not die even when dealt sufficient damage, such as the Frenzied Berzerker, die anyway if dealt more than twice their hitpoints in fire damage from the sun.
Passing closely to the sun can also be hazardous to one's health. An intense aura of heat extends around the sun, dealing half the damage of touching the surface every mile away from the sun you get, until it deals no fire damage at a distance of 15 miles away. The sun also emits radiation. Anything within 15 miles suffers 1 point of radiation poisoning per round, which deals strength, dexterity, and constitution damage. Each mile closer to the sun deals an additional point of radiation damage per round, dealing 15 at the surface of the sun, and then 30 at the core.
(Damage information from Absolution Part 1, a web supplement to the Immortal's Handbook)