Additional points of consideration, as a renewing force, the Tarresque likely has to sever the links to the planes as it goes, to give the world a clean start, and at least a few centuries between recolonization from the planes. Give it time to return to a primal garden.
This also serves to heighten the apparent threat, where the Tarresque succeeds(and it should do so quite often!), the only word of the disaster is from planar refugees, who cannot return to their former world.
Good thought, actually. This
also provides an out to "why don't we just ask outsiders what happened" - they don't know, because they were cut off at the time and for a good long while after.
This gives some weight to the possibility of using Seal of Binding as its sign, or perhaps a modified version of it.
Getting to these a bit late:
Got a few more ways to release the Big T.
The Players: Get a bunch of new players in and trick them into waking the Tarrasque while dangling the non wakey Tarrasque option in front of them.
It's own Essence: Say the Tarrasque is as old as the world. Bits of it will have fused with the land, vegetation and creatures. Creating a whole new species, perhaps something new or maybe another iconic creature.
The Inevitables: The world has gone too long without feeling the Tarrasque's fury and the forges can't create Marut fast enough. Kolyarut are sent to scout ahead and gather information. While the Marut serve as the ones who awaken and free the Tarrasque, perhaps become part of it's army.
Arrogant Villain: In a similar situation to the aforementioned hunter, this guy would release the Tarrasque for their own personal gain. Though the Tarrasque is likely to just eat them and then rampage.
There was a villain who did this and succeeded though. It was a Psion who swapped minds with the Tarrasque and usually walked around transformed into his original form with a custom magic item to hide the tarrasque from spells like True Seeing.
His original body with the Tarrasque's mind was left in an asylum being cared for by Monks.
The players works, but is dependent on the players doing what you want them to do, which is always a risky proposition. It also doesn't give you a villain other than the Tarrasque, which makes it slightly harder to build a plot around other than "you released it, now stop it".
The problem with its own essence is that, as above, you don't have an easy plot hook villain.
The Inevitables works, though I'd personally rather invent a new kind of Inevitable, since the Maruts seem kind of a stretch to apply to this. They could work, though, if there's, say, some druid sect devoted to upholding a seal on the Tarrasque, causing the world to exceed its lifespan.
The arrogant villain is a classic and can come in many forms, really. The hunter and the opportunist are more like subtypes of it.
