This. The Tarrasque is neutral for a reason. It destroys, but isn't more evil than an earthquake or a volcano is. It spends most of its time slumbering, sometimes going into a rampage to shake things up around the world, and then goes to sleep again. The more puny sentient beings try to "tame" the world, the bigger chance the Tarrasque awakens and crushes some civilizations for good measure (all those unlooted buried ruins have to come from somewhere after all).
as a basis as far as special abilities are concerned.
And for your viewing pleasure, I release you the first draft of my Tarrasque monster class abilities. Altough I deemed its full array of abilities too OP for an actual player, they may do just fine for a final boss:
HD:d12
Saves: All good
Bab:Full
Features:
Ancient body: The tarrasque loses all other racial traits and becomes a medium sized magical beast with all the related traits(basically darkivision 60 foot and low light vision). It has a base speed of 40 foot and a natural bite attack dealing 1d10+1,5 str modifier damage, and the scent ability. It has claws and tail altough they're too weak to do anything for now. It's unable to do fine manipulation but it can talk in a gutural deep primal voice.
It also gains a bonus to natural armor equal to his Con modifier+2. Whenever the Tarrasque grows in size it's natural armor grows by an extra +1.
Unless otherwise noticed all of the Tarrasque abilities are Ex abilities, even those that replicate spells.
Assimilation:The tarrasque is an avatar of doom, seeking not only destroying everything in his wake but growing stronger as he does so.
As a fullround action the tarrasque may eat an unatended piece of magic equipment, in wich case it is destroyed and it's essence assimilated into the tarrasque body, granting the magic bonus of the assimilated item (if any) . Magic weapons absorbed transfer their magic enanchments to a natural weapon of the tarrasque's choice (use the natural weapon basic stats). Enchantments that only work on ranged attacks don't benefit the Tarrasque. Armor and shield bonus apply to the Tarrasque's natural armor. Charged items keep their old number of charges and can be used the normal number of times. As usual equipment, bonus of the same type don't stack.
Basically, this ability allows the tarrasque to use equipment whitout it actually equiping it. Bonus gained from assimilation can be supressed by dispel magic and AMF like regular magic items and permanently destroyed by Mormekdain's Disjuction
The tarrasque may remove any number of assimilated item from himself with 8 hours of uninterrupted concentration, in wich case one or more exotic jewels form on the tarrasque's skin, from the condensed essence of the magic items. Those gems can be removed whitout harm from the tarrasque's skin at the end of the 8 hour period. They are worth half as much as the market price of the assimilated items removed.
In adition to the above, whenever the tarrasque needs to make a save against poison or disease, nothing happens if he fails the save, and if the tarrasque makes it then he assimilates the poison/disease and can deliver it with his bite attack. Each sucessfull save only allows for one "charge" of poison/disease, but the tarrasque doesn't need to use it right away, and can hold it indefenetely.
Improved grab:As the srd ability.
Ability score increase: the tarrasque gains +1 str and +1 Con at every level.
Monstruosity:gains regeneration equal to half the tarrasque's HD, wich can only be bypassed by critical hits(including coup de grace). Also gains resistance to fire, cold, sonic, acid and electrecity equal to his HD.
Rush:Once per minute the Tarrasque can multiply his base movement speed by seven for one round as a free action.
Claws: The tarrasque gains two claw attacks dealing 1d4+str damage each, secondary attacks.
Pounce:When the Tarrasque charges it may perform a full attack at the end of the charge.
Growth: The tarrasque grows one size category at levels 4, 8, 12 and 16.
StompAs a standard action a number of times per day equal to his Con modifier the tarrasque may slam the ground he stands in, creating a shockwave that desestablizes his oponents and tears apart the ground itself. The tarrasque chooses either a line with a length of 20 foot per HD and wideness equal to the tarrasque's own space, or a burst with a radius of 10 foot per HD. Anyone in the selected area must make a reflex save (DC 10+1/2HD+Str modifier) or be knocked prone and takes 1d6 damage per HD of the Tarrasque (sucessfull save reduces this damage to half). The affected area also becomes dificult terrain that however doesn't slow down the tarrasque himself.
Multiple uses on the same area stack. So a piece of terrain stomped two times would reduce movement speed by ¼.
At 8 HD the Tarrasque may use this ability as a move action.
At 12 HD the Tarrasque may use this ability as a swift action.
At 16 HD the Tarrasque may use this ability as a free action any number of times per round as long as it has uses.
At 20 HD the Tarrasque can use this ability at will, but only once per round. It adds two times his Con modifier to the DCs instead of just once.
Horns The tarrasque gains two horn attacks dealing 1d6 damage each, secondary attacks.
Begginning of the end: the tarrasque's skin starts to srugg off both sorcery and sword. The tarrasque gains SR=11+HD+Con modifier and DR/- equal to half his HD.
Terror from below: the tarrasque lurks under earth or water, waiting for his oportunity to strike. It gains a swim speed equal to his base speed and a burrow speed equal to half his base speed. In adition the tarrasque no longer needs to breath and it can eat anything as sustenance, as well as gaining tremorsense with a range of 10 foot per HD.
Doom: As the Tarrasque grows it resembles more and more an avatar of destruction. The first time the tarrasque attacks or charges each round, every creature of the same size or smaller than the Tarrasque that can see it must make a Will save DC 10+1/2 tarrasque's HD+Con modifier or be shaken for 1 round per HD. Failing the save a second time while shaken makes the creature panicked. Even creatures immune to fear can be affected by this ability as nothing is truly safe from the tarrasque, but they gain a +5 bonus on the save. On the other hand, creatures gain a -1 penalty on their saves for each size category they're smaller than the Tarrasque.
Primal Roar:The Tarrasque can roar with such strenght it creates waves of impact that bring down everything above him. A number of times per day equal to his Con modifier, as a move action the Tarrasque can roar, affecting all creatures in a range of 100 foot per HD (the tarrasque may reduce this radius if it wishes). The primal roar extends to the ethereal plane itself affecting ethereal creatures.
Flying creatures (and objects like airships and flying islands) affected by the primal roar are mercilessly brought down to the ground at the begginning of their turns, falling prone and taking apropriate falling damage depending on how high they were (ref save 10+1/2 HD+Con modifier for half damage and landing on their feet). Creatures may try to take to the skies again after landing (having to get up first if prone) but they'll be brought down again at the begginning of their next turn taking falling damage again. This affects even the Tarrasque himself of he somehow was flying.
Creatures on land other than the Tarrasque must make a Fortitude save DC 10+1/2 HD +Con modifier or have their speed reduced by half as they struggle against the mighty roar. If they fail the save by 10 or more they're pinned instead. Creatures take a -1 penalty on this save for each size category they're smaller than the Tarrasque.
The primal roar keeps echoing and every creature affected suffers the same effects for 1 round per Tarrasque HD after it roars. If the Tarrasque is in zone where there is no discernible “ground” like the elemental plane of water then creatures fall towards the Tarrasque itself slamming on it's hard body but whitout dealing it damage.
At 12 HD the Tarrasque may roar as a swift action. At 18 HD it may roar as a free action any number of times per day it wants and it adds twice his Con modifier to the DCs instead of once.
Reality bend:the tarrasque's sheer presence is enough to start making the very existence around him twist. The tarrasque gains a permanent mindblank effect and can use plane shift as a SLA 1/day for each 4HD it has, save DC 10+1/2 tarrasque's HD+Con mod.
In adition, the tarrasque can enter personal extradimensional spaces like rope trick and mage's magnificent mansion. Even if the entrance size is too small for the Tarrasque to fit it can tear it open untill it's large enough simply by moving trough it.
Augmented senses:The Tarrasque eyes and ears are exceptionally sharp. It adds his Con modifier to spot and listen checks and benefits from a permanent see invisibility effect.
Devouring bite:The tarrasque can try to swallow a grabbed opponent two or more size categories smaller than it by making a successful grapple check. Once inside, the opponent takes 1d8+1/2 str modifier points of crushing damage plus 1d8+Con modifier points of acid damage per round from the tarrasque’s digestive juices(both damages increase acordingly when the tarrasque grows in size). A swallowed creature can cut its way out by dealing damage to the tarrasque’s digestive tract (AC 10+Con modifier) equal to his HD+Constitution score (not modifier). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out. The tarrasque’s gullet can hold two creatures two size smaller than him, or eight creatures three sizes smaller than him, or 32 creatures four sizes smaller than him, and so on.
In adition, the tarrasque now ignores freedom of movement effects when using this and its improved grab ability.
Reality tear: the tarrasque's natural weapons now ignore all DR that can be bypassed by something. DR/- is only half as effective, and the Tarrasque natural weapons now benefit from a permanent ghost touch effect.
Dimensional Colapse:All teleportation becomes extremely hazardous when performed near the Tarrasque due to his sheer presence. Anyone trying to use a teleportation effect/spell/ability to or from 10 foot per HD away from the Tarrasque must make a Concentration check 10+HD+Str modifier or the effect fails and the one trying to teleport takes 1d6 damage per Tarrasque HD. Even if the save is made the teleportation effect goes awry unless they suceed by 10 or more. Roll a 1d100. The distance traveled by the awry distance is reduced by that percentage (minimum 5 foot reduction).
Destroyer: the tarrasque's natural weapons evolve to deliver gruesome and crippling wounds. Oponents hit by the following natural attacks of the tarrasque suffer the following penalties, fort save equal to the damage taken from the attack to resist, and last untill the target is healed back to his full HP unless otherwise noted. All the abilities down there ignore freedom of movement.
Bite: maimed, take a -1 penalty on all d20 rolls for each 10 points of damage taken by the attack, stacks
Claws: rended, take -1 penalty on an ability score of the tarrasque's choice except Con for each 10 damage taken, stacks (but can't reduce below one)
Horns: impaled, loses dex bonus to AC and can't move anymore but if the tarrasque moves the impaled target is moved along. Can try a new save every round to pull himself out. While impaling, the tarrasque can only use the respective horn to attack the impaled creature itself. A creature of the same size or bigger than the tarrasque can only be impaled if the tarrasque hits it with both horns on the same attack and the creature fails both saves.
Immortal vigor:The tarrasque's regeneration can now heal any ill effect. If the tarrasque would recover any amount of HP thanks to his regeneration, instead he may choose to remove the same amount of negative levels, ability drain, ability damage or a spell/power/supernatural ability effect on him. So for example a tarrasque with 10 HD and regeneration 5 could remove from himself one 5th level spell, or a 3rd level spell and two negative levels. If an effect doesn't have a listed level, it is half of the HD of the creature that originated it.
Carapace: The tarrasque’s armorlike carapace becomes exceptionally tough and highly reflective, deflecting all rays, lines, cones, and even magic missile spells cast from more than 30 foot away from the tarrasque. There is a 50% 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.
In adition, the tarrasque gains DR equal to it's HD (stacking with it's normal DR) against any ranged attacks coming from more than 30 foot away from it.
Prevail:The Tarrasque revels on violence and destruction, even when inflicted upon itself. Specially when inflicted upon itself.
If the Tarrasque is droped to 50% of his normal HP it may add his Con modifier to his DR, SR, saves, skill checks, regeneration, resistances, ability DCs, attack and damage rolls even if it was already adding them.
If the Tarrasque is droped to 25% of his normal HP it may add his Str modifier to his DR, SR, regeneration, saves, skill checks, resistances, ability DCs, attack and damage rolls even if it was already adding them.
Tail Slap:The tarrasque gains a tail slap natural weapon dealing 2d8 base damage, secondary attack. Whenever the Tarrasque attacks with it's tail it may attack once any creature smaller than itself whitin his melee range with it but doesn't gain extra attacks from cleave or similar abilities from oponents droped in this way.
Unstopable Force: created for destruction, the tarrasque tramples over all defenses. The tarrasque can ignore any obstacle on it's way that has less hardness than his HD when moving and attacking. So if for example he detected a creature hiding behind a wall of stone with his tremorsense, he could simply charge trough the wall of stone like it wasn't there.
This ability also allows the tarrasque to ignore all cover, and can even punch trough magic effects that hamper movement like solid fog or even prismatic wall. When facing any area magic effect that would hamper or block movement the tarrasque can make a dispel check against that effect using his own HD as CL, no limit, as an attack action. Force effects can be destroyed by this ability but the tarrasque takes a -5 penalty when trying to dispel them. If the check is sucessfull, the attack punches trough and can be directed at any oponent now whitin reach.
Augmented critical: All of the Tarrasque natural weapons no have a 18-20 threat range and deal triple damage in a sucessfull critical.
Ancient Heart: The Tarrasque's regeneration can't be bypassed by anything, and any effect that would kill it outright instead simply deals it nonlethal damage equal to it's total HP +10.
To kill the tarrasque one needs to leave it uncoscious by dealing enough non-lethal damage and then cast a limited wish to finish it off, but the Tarrasque still gets a last Fort save to resist this.
At 18 HD it takes a wish or miracle instead of limited wish to finish the Tarrasque.
In adition the Tarrasque no longer fails saves on a natural 1.
Earthquake: The Tarrasque's might is such that it can crack the earth itself simply by stomping it's feets as a standard action. This works as an earthquake SLA, useable a number of times per day equal to the Tarrasque's Con modifier and using Str for the save DC. The Tarrasque itself suffers no ill effects from it.
Rend Magic:The Tarrasque's natural weapons tear apart everything, even the strongest fabrics of magic. Any creature hit by the Tarrasque's natural weapons is targeted by a dispel magic effect, CL equal to the Tarrasque's HD, no limit like the normal spell. The Tarrasque may also attack any ongoing effects to dispel them in this way, not only ones that block/hamper movement.
Crush:When the Tarrasque moves over creatures/objects three or more size categories smaller than himself (that normally wouldn't block it's movement) it can choose to crush them. The targets may choose to don't perform attacks of oportunity for the movement and instead do a Reflex save (DC 10+1/2 HD+Str modifier). Failure on the save or performing the attack of oportunity means the creatures are pinned, in adition automatically taking 4d6 damage +1,5 Str modifier(changes acordingly to the Tarrasque's size). No creature or object can be crushed more than once per turn.
Primal Sight: The Tarrasque now benefits from a permanent true seeing effect, except that it extends an extra 5 foot per Tarrasque HD.
Break:The Tarrasque attacks grow even more potent. Anything hit by one of the Tarrasque's natural weapons takes a -1 penalty to AC untill fully healed/repaired, multiple hits stack.
Counter:Whenever a creature inside the Tarrasque's reach attacks it in any way, the Tarrasque may strike back with any of it's natural weapons right after the first attack is resolved
Sky Break: The skies themselves cry when the Tarrasque aproaches. This works as a Storm of Vengeance SLA useable a number of times per day equal to the Tarrasque's Con modifier, except the Tarrasque can start it as a standard action, doesn't need to keep concentration to maintain it, it can last up to one hour per HD and the Tarrasque is completely immune to all the ill effects of the Storm, including being able to perfectly see trough it. The Tarrasque can produce multiple storms one after the other but if they're on the same area the effects don't stack. Save DCs are Con based.
Eruption:Even the land itself screams at the Tarrasque's coming. A number of times per day equal to it's com modifier, as a fullround action the Tarrasque can roar while slamming the ground, making the earth start to tremble. 1D4+1 rounds later a torrent of lava erupts on a place whitin view of the Tarrasque of it's choice. It is a cylinder with a radius up to 5 foot per Tarrasque HD reaching up to 100 foot high per Tarrasque HD. Creatures inside the cyliner get a reflex save DC 10+1/2 HD+Con modifier to get out of the area on time once it explodes. Anyone left inside the cylinder is treated as immersed in lava. This lava is specially hot and burns even creatures with resistance to fire, altough the resistance still applies to the total damage dealt, except the Tarrasque itself. The eruption lasts 1 round per Tarrasque HD.
Apocalypse:The true purpose of the Tarrasque comes into being. To slay all the gods and their creations. The Tarrasque becomes fully immune to all salient divine abilities and epic magic, and can freely use his reality bend ability to enter any private realms (assuming it has heard about them). The Tarrasque also becomes immune to artifacts and can destroy any such trinket by swallowing them.
Creatures slain by the Tarrasque's natural weapons or offensive abilities are utterly destroyed and can't be brought back to life/existence by any means known, even divine intervention (specially divine intervetion).
Add bigger numbers on DR/HP/Stats/Regeneration at your leisure.