Deuce Traveler
Adventurer
As a DM, I treated the tarrasque as a doomsday device. The party's land was being swarmed and destroyed by an invading orc army. They had an adventure where they infiltrated deep into orc lands, fought their way into some forgotten catacombs in a dormant lava mountain next to several important orc military villages, then performed a ritual which caused lava to flow into the center of the mountain in order to wake the 'Lord of Fire Mountain'.
They booked it out of the mountain as it began to collapse around them. When they got out they saw the 'Lord of Fire Mountain' break loose from its mountain. It was a tarrasque, it was pissed, and it proceeded to tear apart the orc lands so badly that the orcs had to stop their invasions and rebuild their society.
So they encountered a tarrasque at level 4/5 but did not fight it.
They booked it out of the mountain as it began to collapse around them. When they got out they saw the 'Lord of Fire Mountain' break loose from its mountain. It was a tarrasque, it was pissed, and it proceeded to tear apart the orc lands so badly that the orcs had to stop their invasions and rebuild their society.
So they encountered a tarrasque at level 4/5 but did not fight it.