A hidden cultist island. Just was watching the wickerman and it reminded me of your post.

The Wicker Man (1973)

The man who went there by his own free will was the perfect sacrifice.

Killing me won't save your damn apples.

The island's name Summersisle.

How does the movie end?

Just prior to the May Day parade, Howie ties up MacGreagor (Lindsay Kemp), the innkeeper who is to play Punch the Fool, and steals his costume.

So disguised, Howie joins the parade led by Lord Summerisle disguised as the man-woman.

The parade winds through town, ending up on the beach where a libation is made to the gods of the sea. Summerisle announces that a more dreadful sacrifice is to be given to the gods who command the fruits of the earth, a horn sounds, and everyone turns to see Rowan tied to a post in front of a cave.

Howie leaps forward, cuts Rowan free, and leads her into the cave. Rowan says that she knows a way out.

After a brief chase, they emerge on a bluff overlooking the sea where Summerisle, Willow, and Rose (Diane Cilento) stand waiting for them.

Howie is stunned when Rowan runs forward and hugs Summerisle, asking him whether she did it right. Howie looks up to see that he is surrounded by locals on the hill above. 'Welcome, Fool,' says Summerisle.

'You have come of your own free will to the appointed place.' He explains that the failure of last year's crop has made it necessary to make a sacrifice to the Sun god and Orchard goddess.

In good times, animal sacrifices are acceptable, young children even better, but not nearly as effective as 'the right kind of adult'...a virgin who acts like a king but represents the law and comes to the sacrifice of his own free will.

He further explains how the islanders lured him here with claims of a missing child and made him believe that Rowan was going to be sacrificed when it was he they wanted at the time.

Against his protests, Howie is stripped, anointed, and dressed in a ceremonial robe.

He is then led up the hill to an immense, 'man' made out of wicker, filled with animals, and sitting atop a pyre.

Howie is forced into the wicker man, the door tightly closed, and the pyre set on fire.

As the fire consumes him, Howie prays to Jesus Christ while the villagers sing.

In the final scene, the wicker man's burning head falls off, revealing a blazing sun setting over the water.