****ATTENTION****SPOILERS AHEAD****
Here's a summary of DDA01 Arena of Thyathis. Although some names have been changed, the text comes almost directly from the module (I'm not sure I have the right to do this, as the text may be protected by copyright. If I don't just let me know...)
Background
Octavio Ostero, a powerful Merchant Prince, with the help of the sorceress Kephira, has been using the Stygian drug zonga to manipulate the arena gladiators, on whose matches great sums of money are bet. As far as Ostero is concerned, the plan works excellently; but he does not realize all its implications.
The gladiators are the bearers of an ancient secret: That there exists an ancient cavern network of unknown proportions right beneath the teeming streets of Messentia. Gladiators who are members of the Order of the Sands are sworn to the guarding of this dungeon’s entrances, to prevent the inhabitants of the underworld from ravaging the city; the greater Order assists in this project, particularly by supporting the legal and financial interests of gladiators. Since the zonga drug entered the Coliseum, chaos has followed. Besides skewing the arena contests, it has led to the general degeneration of the gladiators and guards. Addicts are virtually enslaved to Kephira, who provides them with their drug; in exchange, they are instructed to introduce specific individuals to the drug—preferably before crucial matches. This often results in a new addict; or in the gladiator’s death, when he attempts to defend himself on the sands. What has caused more concern among the gladiatorial ranks, however (since most remain naive about zonga’s abuse), is the increase in monstrous incursions from below. Many recent deaths attributed to monster attacks have been zonga-connected as well, but there has in fact been much more activity beneath the coliseum. Some gladiators fear that the restiveness indicates an impending invasion from below.
One person who has lost a lot of money and gladiators is Proximo, a man of many talents, who some would call a criminal. Proximo is actively seeking vigorous new gladiators to recoup his losses and replenish the ranks of his sponsored fighters; and he is increasingly willing to transgress laws even more than usual.
Adventure Synopsis
The characters begin in Messentia. They are invited to a lavish feast thrown by Octavio Ostero, the head of one of the most wealthy merchant families in Messentia.
Willingly or not, they are impressed into gladiatorial service by Proximo. The Messantian is certainly breaking the law—but he has the connections to give it all the appearances of legality.
The first task that the new gladiators must surmount is their own initiation, which consists in depositing them in the caverns beneath the Coliseum, to see if they can find their way out alive.
After this the party is accepted into the gladiatorial ranks, and they see the day-to-day life in the Coliseum: training, the games, and the odd incursion from below. When a fellow gladiator falls ill, the PCs must take his place—and they win!
That night the characters find their cell left open: they are given the opportunity to escape! In fact it is a plan to have them captured and punished as runaway slaves, in retribution for their arena success. The gladiator who was ill was meant to take a fall, and the characters’ success caused Octavio Ostero to lose a healthy wager. Thugs hired by Kephira (on Octavio’s behalf) subsequently waylay the party.
Things come together at the domus (house) of Proximo. Kephira is to be found there, working out plans to double-cross Octavio, swayed by Proximo’ promises of greater wealth and power, and a chance to grow the precious zonga drug in Messentia itself . . . beneath the Coliseum. When the character party cleans up at the Domus Proximo, they have no reason not to think that they’ve wrapped things up. Ostero remains in the background, however, shrouded at the root of all the intrigue.
The adventure is well written (as most of Mr Nephew products) and needs only minor changes to be adapted to the Hyborian world. The "dungeon" part (through rather small in the original adventure) needs some rewriting.
I was thinking to replace the classic D&D monsters with a race of cave dwellers, degenerated offsprings of some older race... For the rest, a few name changes, some stat blocks and you're in...