Earth-818793 was named by Marvel Comics when Ash Williams met the Marvel Zombies in the Marvel Zombies vs. Army of Darkness limited series. Although Ash's home universe wasn't mentioned by number in the crossover itself, it was officially numbered in one of Marvel's official handbooks. The Earth's number itself follows Marvel's tradition of numbering it's universes after significant "real world" dates, 818793 represents the years that the films that make up the original Evil Dead trilogy were released: 81 (The Evil Dead), 87 (Evil Dead 2), and 93 (Army of Darkness).
Earth-818793's history has been altered on multiple occasions thanks to the actions (or sometimes lack thereof) of Ash Williams. The first instance of Ash re-writing history (and the one that caused the most significant changes) occurred in the second volume of Army of Darkness during the Long Road Home story arc, where he had completely undone everything that had happened in his life post-returning from 1300 AD. This causes several continuity errors across multiple Dynamite titles and characters, with most of these errors never being resolved.
The 1993 film Army of Darkness is an actual film on Earth-818793 inspired by "true events". This was first mentioned in Army of Darkness vs. Re-Animator#1, where it said that shortly after his arrest, Ash told his story about his battles against the Deadites during his trial, and Hollywood (wanting to capitalize on all the media coverage that Ash's trial was getting) quickly made two television movies about these events. Later in Freddy vs. Jason vs. Ash: The Nightmare Warriors(which takes place roughly two or three years after the events of Army vs. Re-Animator), two characters are watching Army of Darkness in a movie theater, which implies that there are now three films based on Ash's life on Earth-818793.