History of how the Tarrasque happened in Mystara...
by Jerry Hovenanian6000 BC - Immortals of Entropy create the Tarrasque in order to spread chaos and death. Thanatos, Dagar(now dead), and Quarz (also dead) were blamed by other immortals for it. but this was never proven.
5800 BC - The Tarrasque wipes out an entire island of Oltecs. Many Immortals try to stop it but it feeds on pure Entropy.
5400 BC - The Tarrasque destroys and Oltec village and a temple of an Immortal named Azta who was angered. She could not destroy the beast but she could take revenge on its creators. She killed Dagar but died herself killing Quarz. Thanatos was now the happiest immortal, not only had his creation killed many people but it was responsible for the death of 3 immortals.
5200 - 3400 BC The Tarrasque continues its wrath Staying active for 2 months of killing then sleeping for 200 years or so.
3400 BC Blackmoor meets the Tarrasque and with all there advances can only drive it off, not kill it.
3200 - 2000 BC The Tarrasque continues its wrath. Staying active for 2 months of killing then sleeping for 200 years or so.
1800 BC Nithia suffers at the hands of the Tarrasque. Realising that their bronze weapons wouldn't even scratch the beast they start making them out of iron from then on.
1600 BC The Dwarves in the Northern Reaches Have a run-in with the beast. They survive by staying underground.
1400 - 1000 BC The Tarrasque continues its wrath. Staying active for 2 months of killing then sleeping for 200 years or so.
800 BC The Alphatian are attacked and drive the beast south.
600 BC The Tarrasque attacks the Milenians.
400 BC - 600 AC The Tarrasque continues its wrath. Staying active for 2 months of killing then sleeping for 200 years or so staying on Davania.
810 AC It swims from the Hinterlands and settles down on the Isle of Dawn for a nap.
The timeline above can be used in almost any campaign to get the Tarrasque into Mystara from 1000 AC - 1020 AC. what follows is what happened IMC.
1014 AC The monster is awoken and starts to feed. A Glantrian mage, remembering his course in planar physics form the mage school remembered rule #1: Never EVER put 2 extradimensional spaces inside of each other. He and his friends quested for 2 bags of holding and made a contraption that would drop one into the other when the rope was severed by a magic missile.
1015 AC There is a very upset
Tarrasque wandering around the Ethereal Plane :)