Our heroes were on an ocean voyage when they were asked for assistance from Blibdoolpoolp, ancient goddess of the kuo-toa (whom everyone just called Blibbidyboop or Bibbity-Bobbity-Boo or Betty Boop). It seems that a covey of sea hags had hijacked a shrine of her fish-faced worshipers and corrupted them to the service of the demon lord Dagon, who wants to ascend to god status. To that end, his new followers had captured a wounded dragon turtle – a creature Blib. considered sacred – and planned to sacrifice the beast in a ritual meant to elevate Dagon to godhood.
The PCs had reservations about aiding an evil, insane goddess with the body of a human woman and the head and hands of a lobster – but she convinced them that while she just wanted to hold onto her scattered worshipers, Dagon would, if successful, drag the world into the Abyss with him. That, and she vowed to send their ship to the briny deep if they refused.
So the PCs followed her urging to a lonely island where the dragon turtle was chained in an undersea cave. They fought crazed kuo-toa (Monster Manual V), skeletons, a stone golem and one of a covey of sea hags, and found a portal that allowed them to bypass much of the dungeon (which is well, because we were running behind schedule before I had to go to work!). They valiantly freed the dragon turtle and received much booty from Blib.
Then, as they were sailing away, they saw the island, which was shaped like a shark’s fin, rise and reveal that it was in fact the fin of some titanic creature that dove back into the sea, creating a maelstrom that threatens to engulf their ship.