Here's the challenge: I, a level 17 Bard taught by the College of Compulsion (a homebrew subclass), am hired by some random bumpkins to go and get rid of the Tarrasque all by my lonesome, for 5 silver pieces. I accept and trek my way over there.
Once there I cast Phantasmal Force at the beast as many times as I need to in order to wear down its 3 legendary Resistances - up to 16 times - whilst leading him to a nearby beach.
After all of its resistances are blown down the drain I pull a final Hail Mary and cast True Polymorph upon the foul beast. Let us say that my d12 bardic inspiration was enough to transform him due to his failing his saving throw.
I turn the horrible beast into a chest-sized stone boulder, light enough for me to carry without too much hassle.
I take him to the waters of the beach I've been leading him to during our scuffles together. Having hired a friend to wait by the beach with a quick ship in hand, we set off for the deepest part of the ocean, the unnamed trench. There I drop the chest sized rock into the ocean to sink to the bottom of the sea. After which my friend and I book it the hell out of there, like there were devils at our heels.
Will the Tarrasque suffocate underwater and die?
No where in all of the Monster Manuals I've studied does it say that the Tarrasque can breathe underwater, or swim. In particular the 2nd edition monster Manual said, it could not be chopped up normally otherwise it would just regenerate.
I thus ask, as a related sequel to the basic question: if the beast was slain without so much as making it bleed, can it ever truly hope to regenerate, and can I truly claim those 5 silver pieces, or should I just get my wizard friend to wish it away with children's dreams?
Clarification: I am using 5th edition, but I've seen that people all across multiple forums use the Tarrasque's regenerative ability from 2nd edition. I am unsure whether or not that ability was still considered canon in the Tarrasque's lore: I felt that I needed to include that as part of the problem to solve.