Tarrasque

Only one tarrasque is known, and that is a mercy. No creature born on the Prime Material Plane is more feared: a scaly, two-legged reptilian giant with a pair of horns, a whipping tail, and a shell that gleams like a mirror.

When awake, the tarrasque strips the land for miles, eating every animal and plant it finds. In a fight it uses two foreclaws (1d12 each), two goring horns (1d10 each), a tail sweep (2d12), and a 5d10 bite that also works like a sword of sharpness, lopping off a limb on a natural 18 or higher. Normally sluggish, once per turn it can charge at movement 15, doubling its horn damage and trampling everything in its path for 4d10.

Simply seeing it paralyzes creatures under 3 levels or Hit Dice with terror, no save, for as long as it remains in view. Anything with 3 or more levels or HD flees in panic, except that creatures of 7th level or HD and up may save vs. paralyzation to resist, and many run anyway.

Its shell makes rays and bolts, such as magic missile, lightning bolt or cone of cold, totally useless; one in six rebounds straight at the caster with full effect, and the rest glance away. Heat and fire cannot harm it, psionics have no effect at all, and only +1 or better weapons can wound it. It regenerates 1 hit point every round. (Note that creatures with a negative THAC0 are hit only on a 1.)

Thankfully it wakes only briefly. Usually it rampages for a week or two, then burrows into a hidden underground lair to sleep for 5d4 months. About once a decade it has an unusually long active spell lasting several months, followed by 4d4 years of slumber unless something wakes it. Overall it sleeps about thirty times as long as it hunts.

Legend says the tarrasque can only be killed by reducing it to -30 hit points or lower and then using a wish; otherwise even a scrap of it will regrow the entire beast. Its shell is rumored to be a treasure trove. Treating the upper shell with acid and firing it in a furnace supposedly produces 10d10 diamonds worth 1,000 gp base each. Belly plates combined with its blood and adamantite reportedly yield a metal that master dwarven smiths can forge into 1d4 shields +5, each taking two years, and the dwarves will expect payment.

Everyone hopes the tarrasque is unique, perhaps a horror made by dark magic or by ancient forgotten gods to punish the natural world. Its elemental qualities lead the handful of surviving experts to suspect the evil archomentals had a hand in its making. Where it lairs is unknown, because it rarely leaves witnesses and plants quickly bury its trail. One vanished civilization's final records describe a huge reptilian scourge the gods sent to bring about the world's end, and many believe that was the tarrasque.