What comes from below

Just outside of settlestone A colossal Purple worm has breached onto the surface shortly before dying due to a number of strange and horrendous wounds along its body. The creature has left behind a 10 wide shaft leading straight down into the ground. The local residents are scared and nervous this may be the first of many. As renowned heroes of the region you have been tasked with seeking out what could have driven this creature to the surface, and if there is a lingering threat to make sure it is dealt with.

This will be a Level 15 one shot, NO UA - Magic item allowance to be revealed closer to the time. Please use the standard array or point buy for character builds.