A tarrasque is something of the go-to immune to everything how do we kill it monster in D&D, as such I spend a silly amount of time trying to figure out how to permakill the fucker. Today I’m thinking of the Pathfinder version, my first instinct was to hit it with ability drain, but it’s immune to “permanent wounds” which ability drain falls under pretty easily. I guess that would have been too easy. Considering it’s other immunities there’s basically no conventional way to kill it and keep it dead reliably unless you have an unreasonably kind or somewhat unimaginative DM. For instance the common thing is to blow the tarrasque to bits by some conventional or overly complex means and then set up a bunch of near-indestructible stuff (chains or spikes typically) to prevent the tarrasque from actually being able to live when it regenerates it’s body. The issues I have with that begin with the inability to know where the tarrasque is regenerating from if you fucked it up too bad. If you did a pretty basic job of killing the tarrasque then its dead body will simply come back to life and so all you have to do is pound massive spikes through its head so vital bits can’t regenerate right? Well with what force does it regenerate? If you killed it dead long enough to pound in those spikes you probably have to worry about my first point in the first place but other than that it could potentially just force out any foreign objects via regeneration. Confining the tarrasque in a giant invincible airtight box full of water and stabbing him through with invincible spears so that he can remain in an eternal coma is pretty much very excessive and a pain in the ass.
A couple Ideas that make sense to me and that I haven’t yet disproved:
Cast “Create Demiplane” and make it stick with a permanency spell because at the end of it’s duration it may just shunt all its occupants to the nearest plane. The tricky part here is hitting the tarrasque with a planeshift spell to get him stuck on your demiplane, but that alone is easier than actually killing him, and if you really want him at 0 hp you can make your demiplane literally nothing but water. The fun fact with this one is that as a standard action the creator of a demiplane can kick an occupant out, which basically means you have a tarrasque trapped in a box and you can just spring him right out of there if you feel like watching him eat an entire continent or something.
Setup a “Teleportation Circle” pointed at the fucking sun and hope your GM is nice enough to let that fly, though it’s actually pretty reasonable assuming you do some research on the sun in character. What’s not reasonable is thinking your GM won’t decide that the tarrasque get’s caught in the planet’s gravity and rains down upon you when you least expect it, because teleport doesn’t impart momentum (I’m actually just assuming this) and you can’t reach the sun with it.
“Trap the Soul” may be your best bet, if you can somehow get the true name of the tarrasque you win at life, otherwise you have to fight spell resist and that’s basically it. You have a tarrasque in a gem, even better than trapping it in its own demiplane.
