Just around from Manacle Point (on the nearby coast) is Porthoustock. The cliffs there are carved into grotesque shapes by extensive blasting.
1 The legend is partially correct and a great demon known only as the kirix is imprisoned beneath the sea bed.
2 The legend is a complete fabrication, but there was evil present at one time. Upon the Manacles an ancient, squat temple dedicated to a dark god, can be found. This is the stone church and it was abandoned centuries ago and can be found by anyone who explores the Manacles.
3 The stone church is a deep one temple, with their hybrids quarrying away at Porthoustock to free a star-spawn of Cthulhu trapped there. They are still a long way from freeing the vast beast.