Iruttu (transl. Dark) is a 2019 Indian supernatural horror thriller film written and directed by V. Z. Durai. It stars Sundar C, Sakshi Choudhary and Dhansika in the lead roles and Vimala Raman, VTV Ganesh, Yogi Babu, and Manasvi Kottachi in supporting roles. Girishh G. composed the film's music and cinematography was handled by E. Krishnasamy. The film had its theatrical release on 6 December 2019 and opened to generally mixed-to-positive reviews. It was later dubbed in Hindi and released under the title Qatil Saya (transl. Killer Shadow) Iruttu is a loose adaptation of the 2016 South Korean film The Wailing (2016) by Na Hong-Jin

In Hillstation Sigapurahali at noon, a black figure is seen walking. Six men are killed by an unseen force in hora grass. Police arrive to investigate. Officer Palraj explains to a superior officer there are no traces of evidence at the scene of the crime.

All the victims' neck bones have been shattered but there are no signs of a handprint or murder weapon. Palraj continues to investigate the crime scene; he seems to find a clue. When Constable Kulandaisamy arrives at the SP Office, he finds a woman trying to immolate herself. Kulandaisam and the female constables calm down the woman. At that moment, ominous chanting and whispering sounds are heard.

Police officer Chezhiyan (Sundar C) along with his wife Regina (Sakshi Choudhary) and daughter Diya (Manasvi Kottachi), relocate to the hill station. On the way, the family have a picnic. Their pet dog, Danny, runs into the forest. Chezhiyan finds Danny but Regina feels this is a bad omen for a new start.

Chezhiyan and his family encounter bizarre events. Constable Kulandaisamy helps the family to settle down in a rented house. Chezhiyan notes all of the windows and furniture are made entirely of steel; Kulandaisamy states this is because wooden items were all destroyed by termites. Diya explores the new house and find a large mirror covered in cloth. While checking the reflection, Diya senses a cloth-covered mannequin is moving. As Regina starts preparing a meal, she hears a tapping noise and scolds Diya to stop making it. She finds the noise is coming from the window; she opens it and hundreds of crows crowd their window, scaring them both.

Constable Kulandaisamy returns to the police station, where he receives a phone call in which a woman cries for help and calls his name. He learns the phone had been out of order for the past two days and discusses the incident with his colleagues.

Panicked, they call to Chezhiyan. Chezhiyan and Regina try to have sex and see an old man in rags standing outside their window. The old man will not talk but one of his constables arrives and states the old man is harmless and mentally unwell. Chezhiyan returns to his wife when he gets Kulandaisamy's call for help. He reluctantly leaves for the station to check on his distressed colleagues. In his absence, Regina becomes disturbed when the music system randomly plays music. She unplugs it and turns it around but it plugs itself back in and starts playing music again. Regina tries to sleep but Diya stands at the end of the bed, jumping on her and biting off her neck. Regina wakes up screaming to realize it was a dream.

Chezhiyan calms his colleagues and calls the telephone department to trace the call. In the morning, he gathers information about the killings with the help of Kulandaisamy. He finds the Hillstation has a peculiar name Sigarpurhali of Arabic origin; it is believed to be an ancient Arabic settlement where Arabs used to practice magic. Kulandaisamy also states an Arabic magic practitioner lives alone atop of the hill and is often seen with the ragged old man. Chezhiyan wishes to meet him while Kulandaisamy warns him not to. The practitioner is seen praying and chanting but leaves his home when Cheziyan comes to investigate.