Horror Films Based On A True Story

Whenever the tagline 'based on a true story' appears at the start of a horror film, the excitement levels are raised because the spooky stuff you are about to see actually happened but allowing for artistic licence, some of the films based on real events is quite eye-opening.
The film Psycho about a psychologically disturbed hotel owner who has delusions about his dead mother and keeps her body in the cellar was inspired by Ed Gein, a handyman and farmhand who was arrested in 1957 on charges of committing two murders and digging up the corpses of countless other women who reminded him of his deceased mother. He skinned the bodies to make lamp shades, socks and a suit in hopes of becoming a woman. The Texas Chainsaw Massacre and Silence of the Lambs are also based on the same man.
The Exorcist is taken from a real-life exorcism of a 13 year old boy in America and Jaws is based on events in 1916 when over a 12-day period in July of that year, five people were attacked by a great white shark in New Jersey.
In 'The Hills Have Eyes' a family driving through the Southwestern desert in a Motorhome takes a shortcut that leads them to run headlong into a family of violent cannibals who live in caves in the hills. The Real Story is of Alexander Sawney' Bean, a Scotsman of the 15th or 16th century who reportedly headed a 40-person clan that killed and ate more than 1,000 people and lived in caves for 25 years before being caught and put to death.
Probably the most famous 'Based on a True Story' film is 'The Amityville Horror' where a family move into a house and are forced out after only 28 days by paranormal activity including disembodied voices, cold spots, demonic imagery, inverted crucifixes and bleeding walls.
'Child's Play' is based on a nurse who allegedly put a voodoo curse on author Robert Eugene Otto which transformed one of his childhood dolls into a killer doll and another killer doll in Annabelle and The Conjuring exists and is currently locked up in a display in the Occult Museum in Connecticut with a warning sign to not open. 
The Possession is based on the real haunted dybukk box story where a box is home to a malevolent spirit. Whoever bought that box faced its consequences. Numerous owners of the box reported strange things happening to them.
The Exorcism Of Emily Rose is based on the life of Anneliese Michel who was possessed by demons so strong that her exorcism took nearly an year to accomplish.