Following the release of The Return of the Jedi in 1983 and before the release of The Phantom Menace in 1999, Star Wars -- a major, decades-long franchise today -- was considered yesterday's news. However, in 1991, Star Wars returned to pop culture in a two-pronged attack that expanded the universe. That year saw the release of Timothy Zahn's novel Heir to the Empire and the release of Dark Empire, the comic series that took Luke to the dark side.
Written by Tom Veitch with art by Cam Kennedy and covers by Dave Dorman, Dark Empire takes place six years after the events of The Return of the Jedi. The Rebel Alliance has reclaimed much of the galaxy from the fracturing Imperial remnant, but recently the tide of battle has swung back around in the Empire's favor. They've reclaimed several worlds, Coruscant among them, by utilizing powerful weapons called World Devastators: Mobile weapons platforms/factories that can break down entire planets for use as raw materials to build ships, droids and weapons.
When their ship is brought down on Coruscant, Luke and Lando hold out with the remnants of their crew until help arrives in the form of Han, Leia and Chewbacca. A disturbance in the Force alerts Luke to an oncoming storm that he just manages to warn the others away from, before he and R2-D2 (the most loyal droid in the galaxy) are swept away. They find themselves on the planet Byss -- a stronghold of the dark side -- and come face-to-face with someone the galaxy believed dead: Emperor Palpatine himself.
Preserved via cloning, the Emperor was reborn and once again leads the Empire. This explains their repeat victories, because he is guiding the Imperial commanders from behind the scenes. The Emperor once again makes his offer for Luke to become his apprentice and this time, Luke accepts.
Luke takes his father's place as Palpatine's apprentice because he believes he must understand the dark side of the Force and why his father fell to it. Leia, having come to rescue Luke with Han, Chewbacca and C-3p0, is much more skeptical. She comes to realize Palpatine's machinations extend to her -- and her unborn baby -- as well. Palpatine's clones eventually burn out, rotted from within by the Emperor's necrotic dark side energy. He plans to overwrite the infant's spirit and supplant it with his own.
The heroes escape with help from Luke, but Luke turns out to be an illusion, a projection of himself he used to get his family to safety. He attempts to overthrow the Emperor only to fall to the dark side completely. He acts as Palpatine's enforcer as the Emperor unveils his new flagship: A Star Devastator called the Eclipse. Leia, fulfilling the words of a Jedi prophecy conveyed to her by a stolen holocron, confronts Luke and Palpatine. Freeing her brother and undoing Palpatine, she deals the Empire a devastating defeat.
While the battle is won, the war against the reborn Emperor isn't over. Dark Empire spawned two sequels: Dark Empire II saw Luke struggle with the pull of the dark side despite his redemption. He worked to rebuild the Jedi Knights even as the reborn Emperor continues his plot to recapture all his Empire has lost. The finale arrived in the form of the two-part series Empire's End, in which the Emperor makes a last desperate move to obtain the body of infant Anakin Solo to transfer his spirit into. His triumph is all but assured when a survivor of the Jedi purge named Brand sacrifices his life and has his Jedi spirit drag Palpatine into the netherworld of the Force, never to be seen again.
The Emperor surviving his death through cloning, Leia being trained as a Jedi and elements like Jedi holocrons and survivors of the Jedi purge were later incorporated into Star Wars canon. Much like Palpatine himself, Dark Empire continues to cast a long shadow.