Shrine of the Abyssal Maelstrom
An underwater temple to Dagon located beneath the Lost Triad. The cultists here wielded the Heart of Charybdis, a powerful artifact that allowed them to generate deadly whirlpools. While the abyssal marauders of Drakefang Isle would whet their appetite for carnage on most vessels, those carrying mystical and mysterious treasures of value to Dagon’s cult would be dragged into the depths by the powers of the shrine.
It was a highly lucrative partnership, until one day a passing ship was said to carry something of such great value, that the cult sought to drag it straight into the Abyss itself. After opening a portal at their whirlpool’s bottom, they dragged the vessel down only to find out that the artifact on board at been sabotaged. A planar catastrophe was unleashed that would drag the Lost Triad into the Prismatic Vortex.
Trapped with no new prey, the cults waited. Their vast libraries of forbidden knowledge and their fleet of patchwork submersibles remained largely idle as their leaders schemed a way to escape from this twisted trap . . .
On their first delve into the shrine, the party engaged a number of cultists and freaky fish guys before being forced to withdraw to rest and recover. However, before they did, they learned that the cult within was on the verge of opening a portal out of the vortex . . . and in to the Abyss.
On their second visit, they encountered ramped-up security that summoned demonic reinforcements. However, when it seemed that things had quieted back down, something when horribly awry with the sinister ritual being conducted within. Reality itself was ripped asunder as gashes in the walls of the shrine gave way corrupted jungles and ocean of death.
Fighting their way through the horrors that arose, the party eventually confronted the leader of the cult and forced him to abandon his ritual and face them in combat. Also, cannons.
With the cult subdued, they learned that they had been sabotaged yet again, presumably by his old arch-nemesis in arcane espionage, an agent of Vecna . . .