This first in the Blood Island trilogy of Philippine shot horror movies is my favorite.
Dr. Paul Henderson (Kent Taylor) and his oversexed wife (Beverly Hills) go to a remote island in the Philippines to help the natives. Accompanying them is Jim Farrell (John Ashley), who is going to teach the islanders better architecture and irrigation for their crops.
What they discover is that the islanders have reverted back to their ancestors' ways, which means they are sacrificing women to their god. This is a horrible looking (well, it is a BIT cheesy) monster that rapes and then tears the females apart limb from limb. It's caused from radiation from nearby bomb testing—and has affected animals and even the trees. In several scenes the trees have these octopus-like appendages and grab people with them.
On the island is Esteban Powers, who lives in a mansion and is assisted by a suspicious looking native. Esteban welcomes the group to stay in his home after the natives become less friendly, though there's something suspicious about him as well. It's no surprise when it turns out that he's the one who turns into that monster.
John Ashley falls for the native girl, Alma, and must save her from being sacrificed.