The Rising Dragon
Every time that the PCs go back to town without clearing out The Risen Dragon, add one to the countdown:
1: Dwarven prospectors are seen in town. They have a mysterious map, and they're very closed about where they're going, fearful others will tap into the treasure they're after. If someone can win their trust, get a look at their maps, or just listen in on their conversation, they can find out the Dwarves have a map purported to lead to a buried trove of loot.
2: The Dwarves have broken the seal on the Risen Dragon. A few days later, rumors start going around as the Dwarves have missed a regular supply pickup in town. A good tracker could probably find out where the Dwarves were camping and digging, though.
3: A minor earthquake shakes the region. A day later, a half-starved and fully insane member of the Dwarven expedition appears in town. He's barely coherent, but magic or persistence might get the story of the terrible things the Dwarves found when they broke in The Risen Dragon.
4: The Harbinger, a powerful sorcerer from foreign lands arrives in town, drawn by the Dragon's call. He's scary, mysterious, and smart. He heads out to the dungeon and sets up shop, intent on bringing the Fallen Dragon to life. Perhaps a smart party can figure out what he's up to, stop him, or even raise the Dragon themselves.
5: Another earthquake, this one heralding the rise of the Fallen Dragon. Whether the Harbinger maintains control of the beast once raised, is up to you.