The Tarrasque is an immeasurably powerful monster from the Dungeons & Dragons multiverse and is considered one of the most powerful monsters in the entire setting, as such the Tarrasque is rarely encountered save for the most drastic of scenarios and even the most elite adventuring party is likely to lose against the colossal beast, the being is so powerful entire campaigns may be formed on how to deal with the rise of such a creature or the resulting chaos its arrival (or possibility of said arrival) may cause to an entire realm.
Due to the immense power and size of a Tarrasque many settings claim that only one Tarrasque is created per world and once awoken they will mindlessly destroy and devour absolutely everything - thus the Tarrasque is True Neutral or unaligned, caring not for good, evil, law or chaos. to the Tarrasque everything exists simply to be devoured or destroyed.
In many settings the Tarrasque is a creation of the gods and as such can not be killed by normal means - if adventurers manage to get the monster's health down to zero they will still fail to destroy it unless they succeed in a "Wish" spell as well. if either fail the Tarrasque shall rise again and again, further adding to its infamy as a near-unstoppable force once unleashed into the world.
Like many monsters in Dungeons & Dragons the Tarrasque is also based on real-life mythology, in this case the dragon known as Tarasque.