The Treasure House of Urgaan of Angarngi:
The Book of Antiquities and Lost Treasures was stolen from Lord Rannarsh's estate.  He offered a paltry reward for its return, which the party refused.  Rannarsh had the party followed and learned of their plan to raid the legendary treasure house of Urgaan of Angarngi.  He plotted to ambush them and force them to recover the treasure for him.

Rannarsh's men arrived at the Sinking Lands on horseback a day ahead of the party travelling by sea (alas poor captain Jack Mackerel and the leviathan).  The ambush was quickly thwarted by the party, and Rannarsh's men died or fled.  But Lord Rannarsh had already ventured within the treasure house.  There he was driven to madness, in confusion and fear, by Ulat of Angarngi (a benevolent descendant of the infamous Urgaan) and by the strangely moving walls of the house.  The berserk Rannarsh attacked the party but was cut down.  The bodies of Ulat and Rannarsh were searched briefly, but abandoned when the party fled for their lives as the treasure house tried to crush them.  The house self-destructed in its rage, its death throws causing the Sinking Lands to sink.  A desperate ride across the sinking lands, racing past the prospecting family desperately packing up their raft (you'll never know what they may have had in store for you), and finally a swim finally brought the party to the shores of the Great Marsh.

The party had won the legendary treasure: a great diamond the size of a cat's skull, twelve rubies that glow in the dark, and a scroll case containing a diagram with mysterious lines and circles and writing in an unknown script.