I had a campaign idea based on Salt in the Wound. The players pass through Salt in the Wound, which is much smaller than it once was, then they go do some quest in a nearby cave system. Once they get in, the cave collapses behind them, and they have to fight their way through it. Once they get out, they find out what the earthquake was. Salt in the Wound used to be doing damage faster than the tarrasque could heal, and it soaked up a lot of nonlethal damage. But since it slowed down, it had gradually been healing, until today it finally escaped.

The players just did their first adventure. They have no chance to stop it. At first, their quests are simply to help with the damage caused in the wake of the tarrasque or warning villages that it’s moving towards. As they level up, they might get powerful enough to change its path, or fight off the various factions trying to use the tarrasque for its own ends. And once the players are powerful, they can finally defeat it. And they have a choice between bringing back Salt in the Wound, which is now more necessary than ever given all the infrastructure that was just destroyed, or killing it for good. Or maybe if they really want to, they’d have some shenanigans that let them control it.