Adventure Hook by Arthur Petersen
After the climatic events of Ghoul Island [SPOILER AHEAD], the volcano of Kaza erupts destroying the Elder Influence of Ghatanothoa and the ghoulish cult supporting him. However, unbeknownst to all, the eruption was so severe it created a hole deep at the base of the island under the sea.
This natural tunnel filters to the ocean where the Deep Ones of Ghatanothoa’s cult still thrive. Their Shoggoth may have been destroyed, but now they are coming from an unexpected direction: from the caves below the volcano, not from the sea! Will the Deep Ones try to revive their Elder God? Or simply assault the remnants of Farzeen and try to take control?