*I seem to recall having read something about DR being supernatural if it can be pierced by magic, alignments, or epic, and extraordinary if damage types or materials did the job. Can anyone confirm this?
A breakdown:
Step 1: Determine where the tarrasque is. Be there when it's napping.
Step 2: Manifest astral seed. Die. This is important.
Step 3: Manifest mind switch on the sleeping tarrasque.
Step 4: Kill the spare.
ALTERNATE SETUP, depending on where you go when mind switch ends**:
Replace mind switch with true mind switch and remove astral seed.
**I'm assuming you stay in the current body, since astral seed implies that you can in fact permanently usurp a meatbag by mind switching and then destroying your soul jar.
It's not the optimiziest or the gamebreakiest, since it relies heavily on taking on the body of a creature that's only active for 1d3 days every 6d4 months (ignoring its 1d2 week rampage every decade or so with a 4d6 year slumber) and losing a fair bit of what makes it an effective beast, such as over half its hp (assuming the psion rolled average), probably its DR and SR, and all of its feats. It does, however, shore up a lot of the psion's defensive problems and lets him spend power points more freely, since he can always fall back on bite/clawx2/hornx2/tail slap. Mostly this was just a thought experiment. Theoretically this could work with anything that can fail a Will save and is vulnerable to mind-affecting effects. It's generally not very practical, though, since most creatures have more HD than CR and the ones that don't are ones that don't give you much benefit from body-snatching, generally getting their high CR from non-extraordinary abilities, which true mind switch doesn't let you take.
I wasn't sure where to put this, so I hope I got it right.
The improved overchannel epic feat (Complete Psionic) allows you to increase your manifester level to twice it's normal value. You could pull off a mindswitch with the tarrasue when your normal manifester level is 24, but you'd take 48d8 damage from doing so. (You may want to manifest vigor.) You need to be 22nd level to take the feat. You can probably find a couple of +1s to manifester level and pull it off right when you get the feat.
The errata for the Monster Manual says DR X/magic is supernatural. It doesn't directly address DR X/epic, but I think that would be supernatural as well.
In scheming to take the tarrasque's body, you should get some sort of class-derived immunity to mind-affecting effects. You don't want someone coming along and body-naping what you've rightfully stolen.
true mindswitch a pitfiend's body, its more fun and lots easier to pull off, with a lot less gimping
I'd say that the pit fiend is a lot more difficult to get than the Tarrasque.
Pit fiends: Are intelligent, spellcasting, 24/7 vigilant (they don't sleep eat or drink) fliers with wish and the whole system of a powerfull,lawful,organized (as much as it gets) society backing them up.
There will definately be backlass as punishment for your transgresions in order to set an example for the others that will follow your footsteps.Thats the way these kinds of people work.
On the other hand the Tarrasque: Is a very large, strong, non flying, non spellcasting, nonintelligent, unorganized animal with strong passive defenses, completely no allies to back it up or care for it and the bad habit to sleep...a lot...
Yeah.. I'd go with the Tarasque if I wanted to avoid trouble.
spellcasting = yah so? if I'm going to true mind switch one, I don't really care, or I do care and can shore up my spell defenses before hand.
dont sleep, eat, or drink = that's one of the incentives to pinch its body
fliers = don't care, i can fly too
with wish = once a year... they likely already used their wish when you encounter them, besides, I've got reality revision usuable a lot more often.
powerful lawful organization = devils backing each other up? pfft, only if they have contracts or are under orders from someone holding power over them... usually the only back up the pitfiend has is the minions he's encountered with and any other bodies that get summoned if at all; all of which are easily dispatched or ignored.
why would you want to take the body of a creature that spends most of its life asleep when you can have the body of a creature that doesn't need to eat, drink, nor sleep?
From personal experience, I brought up taking a pit fiend's body would be more beneficial to the character. Gate it in, then cast dimensional anchor on it, beat it down into submission in any flavor you like (i ego whip'd it unconcious), limited wish (bend reality) it to take a -7 penalty on its next will save or fort save, have someone in the party cast greater bestow curse or energy drain on it to set it's wisdom score to 1 or give it a heinous penalty to will saves, bend reality it to take a -7 penalty on its next will save again, then true mindswitch it... it needs a natural 20 to resist at that point. Don't worry about the 600 xp, you'll get it right back for defeating the pit fiend AND you have a nice new body to get used to.
But if you're dead set on getting the Tarrasque, just remember that there is only one of them, and any powerful magics tied to summoning it or gateing it would work on you while you had the body.