Svaha: The Sixth Finger (2019)

In 1999, a woman in Yeongwol gives birth to twins. One, Geum-hwa, is born with a damaged leg, because her twin fed on it in the womb. The other twin is so deformed that the family calls her "it". She is never registered and is expected to die. Sixteen years later Geum-hwa lives with her grandparents, who raise dogs, while her sister, still alive, is kept locked in a shed in the backyard. When villagers come to investigate the strange behaviour of local animals, snakes pour out of the shed.

Pastor Park runs a religious research centre that exposes fake cults, often for money and publicity. He is hired to investigate Deer Mount, a strange Buddhist sect in Gangwon-do, and sends his assistant Joseph to join it undercover. At the same time, police find the body of a teenage girl hidden in concrete. The suspect, a truck driver named Kim Cheol-jin, kills himself before he can be arrested.

A monk friend explains that the sect's four branches represent the Four Heavenly Kings, guardians of the four directions. Park learns that the sect's founder, Kim Je-seok, was born in 1899 and is said never to have aged. Years ago he adopted four young offenders who had killed their fathers and raised them as his guardians. Cheol-jin was one. The last one alive is Na-han.

A Tibetan master tells Park that Kim Je-seok was believed to be immortal, possibly Maitreya, and had a sixth finger. He also delivered a prophecy: a "snake" would be born in Kim's birthplace a hundred years after him and would put out his light. Park realizes the guardians have been killing every girl born in Yeongwol in 1999, and the numbers in the scripture are their registration numbers. Na-han has been stalking Geum-hwa.

Na-han kidnaps Geum-hwa, who begs him to kill her twin too so the sister can be reborn as a human. When he goes to the shed, the sister is no longer monstrous. She sits calmly, like a Buddha, and sings the lullaby that has comforted Na-han in his nightmares after each killing. She tells him to look for the sixth finger on the man he calls Father.

The dying old man Na-han serves has only five fingers. The truth is that he was just a disciple. The real Kim Je-seok is the young caretaker on the grounds, who has stayed young for decades. He shoots Na-han and calmly explains that Na-han was one of the stars protecting his light. As they drive, Na-han attacks him and the car crashes. With fuel soaking them both, Na-han uses the lighter the sister gave him to set Kim Je-seok on fire.

The big question fans ask is who the "snake" really was. The film suggests the twin was never evil. She was the force sent to end a false god, and Kim Je-seok, who killed children to protect himself, was the real devil. Geum-hwa survives and holds her sister as she dies. Pastor Park, who never truly believed, leaves shaken by what he has seen, facing a real crisis of faith.