The Grass Tombs (1977)

So-dol, serving a prison sentence after being accused of murdering his friend, gets a 7 day leave from the prison when his mother dies. As soon as he and the guard assigned to him arrive in the village, strange events begin to occur. A 5-year old corpse is discovered floating in the harbor and the entire seaweed harvest spoils. So-dal's guard begins an investigation and find that the supernatural is very real in this remote island.