From the introduction:
Xarr-Zuul was a powerful priest of the forgotten cult of Hròkrolc (minor divinity related to death, decay and necromancy) buried centuries ago, together with other acolytes, in a tomb under a now abandoned and half collapsed shrine, mostly reclaimed by the surrounding woods. Rumours of horrible curses kept the people of the near town of Ophir away. Until now. Greedy young members of the Merchants Guild, not scared of old rumours, hire the PCs to unveil the tomb’s secrets, promising to pay generously for valuables they bring back. If the PCs successfully pilfer anything from the treasury, however, the real Xarr-Zuul will wake up, get out of his hidden burial chamber and start his revenge. He'll raise the dead bodies in the tomb and call other undead creatures to his protection, then summon devils from the Nine Hells, allies of his god Hròkrolc and send them to murder the people of Ophir. The terrorized merchants will beg the PCs (promising very generous rewards) to go back to the tomb and stop whatever evil power they have awakened. If they defeat Xarr-Zuul, the PCs will gain a new long term enemy in his god Hròkrolc.