Hellstone Deep is a d20 System adventure for four to six 18th- to 20th-level characters. Chapters include an introduction, a three-part adventure ("The Temple of Iron Flames", "City of the Frozen Sea", and "Hellstone Deep"), followed by a afterword, and an appendix with NPCs and a number of advanced, templated, or new monsters (including the bloodscreamer, ebon eye, hellish ripper, and the chthonic vampire and scar twins templates); a new spell (switchback); and new magic items (hellstone brand, ring of messages, and the artifacts black mirror, chthonic heart, iron flames, and vampire pool).

From the back cover:

There was a time when darkness reigned. In a war lost to history and myth, the dread God Chairos engaged in a campaign of destruction and conquest. His goal was to topple the existing pantheon of gods, and to establish a dark new order in which he was supreme...a goal in which he was nearly successful. Only after a long and vicious campaign between Chairos' diabolic forces and the celestial armies of good were his evil plans torn asunder.

But evil never dies. For Chairos' mightiest weapon in his dark campaign—the Chain of Scars—has been discovered. Draz Ghulan, a fiendish general with his mind set on revenge, has rekindled the flames of the ancient conflict with his burning hatred. In his possession lies the Chthonic Heart, the one item capable of reactivating the awesome power of the Chain of Scars. Unless Ghulan is stopped, Chairos' lost campaign of terror and annihilation will begin anew, and the awesome and untold powers of the Chthonic Heart shall be released unto the unsuspecting world.

To stop him, heroes will have to engage in a deadly race against Ghulan's own agents of evil, journeying though the stygian depths of the subterranean city of Il'drazza'kuul and to the horrific demi-plane known as The Sea of Endless Night. Ultimately, their quest will bring them to Hellstone Deep, an impenetrable fortress populated with Ghulan's Hellish minions. But within the Heart lie powers that even Ghulan cannot understand. Powers that, if released, could spell disaster for all. And darkness shall reign again...