Shortly after Disney acquired the Star Wars licenses for Lucasfilm and George Lucas's other related properties, speculation among fans went into overdrive regarding the potential for new movies. After all, there have been thousands of comics, novels, short stories, and cartoons expanding the Star Wars universe, which left a lot to work with. Then, Disney changed nearly everything in the Expanded Universe to be "Legends," which stripped it all from canon. However, the announcement of Episode VII brought back a little hope regarding one series of books from Dark Horse Comics.
Star Wars: Dark Empire was first published in 1991 as a six-issue miniseries, which was followed by two sequels, Dark Empire II (1994) and Empire's End (1995). The books told the story of what happened after Return of the Jedi, so it wasn't a stretch for most fans to assume the new trilogy of films would use them as a template. When The Force Awakens was released, those hopes were dashed, as the new film didn't mirror the events of the Dark Empire trilogy - but that changed with The Rise of Skywalker.
Anyone who read those comics wasn't surprised to find that Palpatine somehow survived the events of Episode VI via a clone of some sort, and that was just one of the events from the books that have now made their way to film. Rise of Skywalker tapped into Dark Empire somewhat, but there was a whole lot more going on in that story.
The original idea for Dark Empire featured an antagonist wearing Darth Vader's armor - or a recreation of it. Vader was a symbol of fear, which would have been used to force worlds on the brink of the former Empire's territory into obedience. The idea had its merits, but when George Lucas heard it, he objected almost immediately.
Afterwards, the concept of cloning Palpatine to bring him back from the grave was given the green light, and while most accounts indicate Lucas himself came up with the idea, he may have only approved the concept once it was pitched to him.
When Dark Empire was still in the design phase, it was going to be published at Marvel Comics, which held the license and had a large library of previously published Star Wars comics. To suit the established comic book canon, Dark Empire was originally set to take place shortly following the Battle of Endor. When the license shifted to Dark Horse Comics, things changed a bit.
At the time, author Timothy Zahn was writing three novels that would become known as the Thrawn Trilogy. He was asked to work in elements of Dark Empire but refused due to issues with the story's timeline. To make the chronology work, the events of Dark Empire were shifted to take place after the Thrawn Trilogy, and the "opening crawl" reflects this by referring to Zahn's books (instead of the other way around).
Operation Shadow Hand is the final push by the remnants of the Empire to retake control of the galaxy. It begins by sending World Devastators to Mon Calamari, the homeworld of Admiral Ackbar, because of the support it has given to the Rebellion in the form of ships and soldiers.
In keeping with the Emperor's penchant for world-ending devices, World Devastators are large machines constructed with two ideas in mind: wiping out worlds and using their raw materials to create new World Devastators, starships, droids, and starfighters. The Devastator's creator, Umak Leth, describes one like so: "Great fires burn in its belly... and everything it eats is transformed into new instruments of doom."
Operation Shadow Hand kicks off with a specific event: the formation of a Force Storm - which is the Star Wars way of saying a wormhole. Luke and R2-D2 are swept up in the Force Storm and taken deep into the galactic core to the world of Byss.
It is on Byss that Luke first meets the clone of Palpatine, who is inhabited by the original Palpatine's spirit. In perfect evil villain fashion, the Emperor explains to Luke that his survival depends on fresh clones, as the dark side of the Force has a punishing effect on his flesh.