The Obsidian Sands of Syncrates (2011)

From the back cover:

Returning from a successful trading voyage to far-away lands, the party's ship is stranded in the doldrums. After two days without any wind, the sy darkens and small particles of black ash fall like a dark snow. The party's two native guide-translators panic and begin yelling wildly, quickly whipping themselves into a fury of fear. Faster than the group can react, the two guides pull their small knives and stab themselves in the chest repeatedly as they jump over the side of the ship. Their dark-skinned bodies unnaturally sink down to the depths below the glass-like surface of the still sea as, one-by-one, each party member feels an irresistible sleep suddenly come upon him.

When the party awakes, it finds itself in what appears to be a giant coliseum, their seaworthy boat stranded upon the obsidian sands that line the floor of the great arena. What foul sorcery has sent you here and how will you return to your world?