Kraedin said:
It is killable, it's just tricky. To get it to -10, you need Slay Living, Destruction, or some other non-ray death spell. Then a simple Bestow Curse to lower it's constitution score by 6. That causes it to lose 48*3 hitpoints, which gets it well below the number we need here, -30.
I'm afraid that wouldn't work. PHB, page 9,
Constitution, first bullet point: "...a penalty can never drop a Hit Die roll bellow one...", and DMG, page 72,
Ability Score Loss, top of second column: " A full hit point score, however, can't drop to less than 1 hit point per Hit Die."
You always have at least one HP per HD, and in the situation you outline above, the tarrasque would just stay at -10 hit points. (No matter how much you drain the tarrasque's Con score, it would have at least 48 hit points, although it's technically dead at a 0 Con.)
That's why I went for the Stirge Swarm of Death (tm), because it's the only way I can think of to reduce it's Con to 0. If that doesn't count as -30 hit points, then the thing is impossible to kill, short of using a
wish to force it to take real damage instead of subdual damage, and then using a second
wish to keep it dead. (If it get's errata'd to Fast Healing instead of Regeneration, then this problem goes away, but until then that's about what it would take. )