Some other possible approaches
"The shipwreck incident" - a 19th or early 20th century sailing ship with some (cult related) goods from Eastern Asia, the Indonesaian archipelago or Africa may have foundered on the rocks/reefs of Inaccessible Island, maybe with one/ a few survivors making it ashore and inland, possibly bringing along the "treasure" (made of precious material ??). The survivors may have degenerated ( Tsotaghua angle ? ) or otherwise have been imbued with longevity and guard the treasure/idol/relic ever since.
The players find manuscripts about a certain "cultish McGuffin" and it being transported on a specific ship, which never reached harbour - cross-referencing finds the ship ( maybe via the insurance company'S files ---> LLoyds of London ?) and the location of its wreckage.....
A whaler from New-England would work very well for this, too, bringing home a cthuluoid "something" from Polynesia or the South Pacific ? Maybe they sailed to Tristan da Cuncha to reprovision/rewater and struck the island in fog, dead calm with sharp currents or in the throes of Madness ??
- "Temple of the Elder Race"..... Some ancient city/Site of the "Elder Race" is located here, providing insight into their schemes and devices ? Offshore observation by a passing ship may reveal strange "inhuman" structuress" in the forests (?) covering "Inaccessible" ? Or something "odd" in the cliff faces, or poking up through the breakers surrounding the coast ? A sailor's log, a harboursidebar-tale or something similar may turn up this tantalizing clue... Or perhaps, the report of some very very unlucky fellows who landed, went insane and had their drifting craft later found along with their half-eaten corpses (cannibalism) by another ship ? Perhaps during WW-I ? And one of these days, the PCs meet up with a haunted survivor of the finding crew who filched a diary or log-book from the corpses ?
