When the PCs then took their Tower to confront the tarrasque, that was indeed what they found. Upon arriving at the tarrasque's location they found the tarrasque being warded by a group of maruts who explained that, in accordance with a contract made with the Raven Queen millenia ago, they were there to ensure the realisation of the end times, and to stop anyone interfering with the tarrasque as an engine of this destruction and a herald of the beginning of the end times and the arrival of the Dusk War.
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I wasn't sure exactly what the players would do here. They could try and fight the maruts, obviously, but I thought the Raven Queen devotees might be hesitant to do so. I had envisaged that the PCs might try to persuade them that the contract was invalid in some way - and this idea was mentioned at the table, together with the related idea of the various exarchs of the Raven Queen in the party trying to lay down the law. In particular I had thought that the paladin of the Raven Queen, who is a Marshall of Letherna (in effect, one of the Raven Queen's most powerful servants), might try to exercise his authority to annual or vary the contract in some fashion.
But instead the argument developed along different lines. What the players did was to persuade the maruts that the time for fulfillment of their contract had not yet arisen, because this visitation of the tarrasque was not yet a sign of the Dusk War. (Mechanically, these were social skill checks, history and religions checks, etc, in a skill challenge to persuade the maruts.)
The player of the Eternal Defender PC made only one action in this skill challenge - explaining that it was not the end times, because he was there to defeat the tarrasque (and got another successful intimidate check, after spending an action point to reroll his initial fail) - before launching himself from the flying tower onto the tarrasque and proceeding to whittle away around 600 of its hit points over two rounds. (There were also two successful out-of-turn attacks from the ranger and the paladin, who were spending their on-turn actions in negotiating with the maruts.)
The invoker/wizard was able to point to this PC's successful solo-ing of the tarrasque as evidence that the tarrasque, at least on this occasion, could not be the harbinger of the end times whom the maruts were contracted to protect, because it clearly lacked the capacity to ravage the world. The maruts agreed with this point - clearly they had misunderstood the timing of celestial events - and the PCs therefore had carte blanche to finish of the tarrasque. (Mechanically, this was the final success in the skill challenge: the player rolled Insight to see what final argument would sway the maruts, knowing that only one success was needed. He succeeded. I invited him to then state the relevant argument.)