The Tarrasque is an enormous abomination, roughly the size of an ancient dragon, with two long horns extending from its forehead, a thick carapace, mighty tail, and a wide, toothy mouth. The Tarrasque also has two small eyes but it does not rely on these for its primary method of sensory perception and if they are blinded or removed, it is effectively unhindered. The Tarrasque is impossible to frighten or charm and has resistances to every kind of damage imaginable. It moves at a speed roughly half that of a human, either while walking on the surface, climbing a height, or burrowing through the earth.
The Tarrasque lives underground, sleeping for several months to a few years at a time. It remains active for only a few weeks to a month. Naturally, you will only find the Tarrasque outside.
More often than not, you will encounter the Tarrasque after it becomes active. It is rare to be at the place where it emerges from its underground lair... and very unlucky. The Tarrasque eats anything; plants, meat, rocks, you name it. It eats non-stop while it is active; effectively turning the surrounding terrain into a barren wasteland: a dead giveaway that the Tarrasque was recently in the area.
The Tarrasque, in most versions of D&D, is either immune to damaging spells, or had an upwards of 85% magic resist, with a 10% chance to reflect the spell back at the caster. The average melee weapon required to strike it is +3 or better.
The Tarrasque has a grounding aura that envelopes it and the surrounding area (roughly 100 yards in any direction). This aura prevents anything from being more than a few feet off the ground, and reduces flying and walking speed by 50%. Oddly enough, this aura does not hinder arrows or projectiles.
When charging or roaring, affected life forms must roll a save. If the save fails, the affected life forms are paralyzed with fear (save ends).
The Tarrasque’s greatest strength are its natural weapons. Every turn, the Tarrasque can attack twice with its claws, once with its tail, and either a stomp or a vicious bite. The Tarrasque’s teeth are incredibly sharp, despite its despicable diet. If the Tarrasque rolls a critical hit with its bite attack, it severs a limb from the target (no save). If it rolls a natural 20 with this attack, the target creature is swallowed whole (no save). The Tarrasque’s senses are incredibly fine-tuned. It is unhindered by blindness, strong smells, or deafening noises. Except in the case of magical concealment (including sound and smell) the Tarrasque has 'true-sight equivalent.
The Tarrasque, once reduced to the appropriate "death" hit points, does not die. In 4th edition D&D, the Tarrasque retreats to its underground lair and immediately goes into hibernation. In 1st and 2nd edition, you must reduce the Tarrasque to a number BELOW "death" hit points, and the use a wish to ensure its demise.
The Tarrasque is the physical equivalent to a demilich. It is called THE Tarrasque for a reason: there's only one. It is nearly indestructible, even harder to kill, and reserved for parties whose only challenges come from combatting deities.
The Tarrasque’s lair has nothing in it, except the Tarrasque. The Tarrasque does not horde treasure, like dragons or liches. Your reward for felling this freak-accident of nature is the equipment you can have crafted from its teeth, claws, hide, and (if you roll high enough on treasure), a slew of coins and equipment from slightly-less fortunate adventurers.
If you plan to combat the Tarrasque, (or are unlucky enough to encounter it above ground) it is HIGHLY recommended that you have 8 or more party members. Odds are that 3-4 of you will be eaten or dismembered, so have enough extra party members to compensate for the loss of damage or healing... this is an incredibly-long encounter.
This creature's origins lie in the French legend of the Tarrasque. The creature was aesthetically changed to better suit the game; but many similarities remain.
In the original version of D&D, the Tarrasque was the most feared creature in existence. It still remains as one of the most powerful and difficult-to-take-down creatures.
It is said, (in D&D), that the Tarrasque was either created by the gods, as a way to punish and remake the world... or by dark wizards as a weapon to combat the gods themselves.
It is said that somewhere there exists a planet inhabited by multiple Tarrasques. On their home world, they are peaceful silicavores (rock-eaters). When they are removed from their home environment, their temperament becomes violent and destructive.
It is said that the Tarrasque lives without purpose, unless that purpose it to eradicate all life. The creature, although incredibly violent, is of neutral alignment... because it lacks the intelligence to make the choice between good and evil.
Armor made from a Tarrasque's hide is said to be as light and flexible as leather, and as durable as +3 full-plate. Despite its light weight, classes that normally wear chain mail or leather still suffer a dexterity loss as if they were wearing full plate. The lower weight is helpful when swimming or flying, and does not hinder movement speed. This armor may or may not (50-65% chance in favor) to retain its spell-immunity after being crafted, and a 30-45% chance to retain the 10% spell reflect.
Many of the Tarrasque’s organs have alchemical purpose. The acid in its stomach is said to eat through almost anything, so bring a magical container if you wish to make use of it. The stomach itself can be crafted into leather armor that has high resistance to magic, and immunity to acid and poison. The Tarrasque's blood is said to have a special, but unknown magical quality to it. Bottle some up, and experiment with it.