The events in the The Evil Dead brought a different kind of demonic possession and an unlikely hero that would become a legend in later films, as well as a book with a mythology of its own. The film follows a group of friends vacationing in an isolated cabin in the woods who discover a tape recorder and a version of the Book of the Dead, which has the power to unleash demons and other malevolent spirits.
Evil Dead 2 retold part of the story and added a different version of the book, now called Necronomicon Ex-Mortis, and quickly became a legend itself, with a number of libraries and pranksters claiming to sell copies of it. The Necronomicon Ex-Mortis doesn’t exist as a physical book but it does as a concept, and had its origin in the mind of one of the most influential horror fiction authors in history.
The Necronomicon Ex-Mortis was taken from the works of H.P Lovecraft. Its first appearance was in the short story “The Hound” in 1922, and since then, many other authors and artists have taken it an added it to their works. In the Evil Dead universe, the book was written by the "Dark Ones”, Eldritch beings who are not demons but are not human either. Their main purpose was to use the Necronomicon as a weapon against humans – and they succeeded. The cover of the book is made of the flesh of the “Damned Ones” and it was written with blood. It’s full of dark ages and spells, the most notable one being that which opens a portal to the underworld so that the Kandarian Demon and the deadites can be released.
The Necronomicon Ex-Mortis also includes information to open a portal to travel either to the past or to the underworld, home of the Deadites. The power of the book is such that it can affect inanimate objects too, as seen in Evil Dead 2. Although there’s a lot of evil and darkness in the Necronomicon, it also mentions “The Chosen One”, that who can fight the evil summoned through it.
Throughout the Army of Darkness showed the Necronomicon from Medieval times, which had spells to go back and forward in time, as well as other versions used to trick those around into thinking they’re the real book. The Evil Dead franchise has taken a lot of liberties with its characters and timeline, and it’s not surprising that the same happened with the Necronomicon Ex-Mortis – although all versions are equally dangerous, so it’s better not to mess with it like Ash has been doing.