Heroic tier - AKA lvls. 1-10
The party, a group of uber-noob adventurers, are tasked to root out an evil cult in a bustling fantasy metropolis. After many trial and travails, they discover the lair of the evil cult far below the foundations of the city.
Disrupting a crucial summoning ceremony, the party barely escapes a dramatic cave-in - blocking their only known means of egress. Their only option is to continue deeper into the earth, to discover a way out.
Paragon Tier - AKA lvls. 11-20
Exploring the subterranean caverns, the party discovers a thriving ecosystem of giant fungi, glowing predators and prey, and fanatical cultists - also escaping the cave-in. Only, the cultists seem to be seeking something down here as well.
Facing down a fearsome creature spawned in the very bowels of the earth, the party discovers a means to return to the surface for rest and resupply - before descending once more into the depths to stop an evil plan from being realized.
Epic Tier - AKA Lvls. 21-30
With their hopes and supplies renewed, the party must storm an underground fortress being used as ground zero for the cult's master plan. The compound is in reality a massive walled city-state, in which the one time citizens are being used as slaves and sacrifices.
The adventure climaxes with an epic battle against a great evil.
223 words that will eat up the next year or more of your life : ) Edition neutral, and including most of the tropes we all expect (and sometimes demand) in our stereotypical fantasy game.