Quote Originally Posted by Mr. Friendly View Post
BBEG finishes ritual *just* as PC's (barely) defeat the villain's minions. Big T goes buck wild and the PCs get to fight him on the DM/BBEG's terms, in a place of their choosing, with depleted resources.

Then Big T is well worth his CR.
No, he's not. Then the PCs are fighting him on someone else's terms, in a place picked by someone else, with depleted resources. That will make ANY fight tougher! It doesn't increase the Tarrasque's difficulty more than it does anything else.