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I had used one as the BBEG in a campaign I ran once. It was strictly for a their tier 4 climax and they had multiple side quests to help get some advantages like overcoming the spell resistance or temporarily stoping the health Regen. However, I also gave the Terrasque a ranged attack, and a few extra abilities so the players couldn't metagame and fly and kite.
And they had to figure out a way to trap the monster in a canyon.
Even still I dropped every PC to 0 health at least once in that encounter before they won. A real nail bitter.
It was one of the top five boss fights I ever was a part of on either side of the screen.
Nice! I had a 3.5 campaign I was running once where one of the major plot points was going to be a guy who was cloning Tarrasques and experimenting on them, but the game fizzled out because of me falling down on the job.
Woah cool idea!