From the back of the book:
The duchy of Thessalaine is under siege. Fishermen and merchants set sail into the Gulf of Irul Kinthé and never return, leaving the port city of Harwynn's Shoal weeping for lost husbands and sons, and desperate for lost revenues. Pleas sent to Ellgrym's court, begging for aid, are ignored, for Madradur, the duke's capital city, has its own problems. There, the nights are no longer safe to walk, the streets and the sewers swarm with creatures from the depths: Grimlocks, orcs, and other, more hideous horrors. And leading them, cloaked in shrouds of darkness, the drow. Nightly, Duke Ellgrym's soldiers are beaten back by these attackers from beneath, and none know why they come or where they might strike next. No men are available to investigate the vanishing ships from Harwynn's Shoal.
One beleaguered province with two very different problems. One stalwartgroup of heroes, who may have bitten off more than they can chew, caught upin them both. From caverns deep beneath the sprawling city of Madradur to tidal caves along the coast of Irul Kinthé, they must follow a trail of spilled blood, old hatreds, and subtle clues - clues that may just lead them to suspect that Thessalaine's "unrelated" problems may not be unrelated at all. Clues that, if they're fortunate, will allow them to solve the mysteries that surround them.
And that, if they are not so fortunate, might lead them into the grasping tentacles of the greatest underground horror of them all.
Shadows under Thessalaine is a D20 system adventure suitable for third to fourth level characters.