Secret of the Great Rift

From publisher blurb:

Investigating the underworld tunnels found around and beneath the Great Rift, you and your companions have pursued the treacherous Derro to discover and stop whatever scheme they have hatched to vex the world of men. Passing through the ruins of S’lero, you have discovered what seems to be the center of the puzzle. But what, or who, holds the secret of the Great Rift?

This adventure is intended for mid- to high-level characters, and is compatible with most old-school games.