Talking About 'Star Wars': Prior to Star Wars: Episode III was there ever a canonical explanation for the appearance of Darth Vader?

The November 1977 issue of Star Wars Poster Monthly revealed - vaguely - how Vader became a cyborg. © 1977 Paradise Press and 20th Century Fox Film Corporation

On Quora, someone asks:

Prior to Star Wars: Episode III was there ever a canonical explanation for the appearance of Darth Vader?

My reply:

As early as late 1977, George Lucas allowed Lucasfilm, through the official Star Wars Poster Monthly magazine, to reveal tantalizing bits of the Dark Lord of the Sith’s backstory.

In an article titled Darth Vader Lives, (Issue #2. November 1977) author John May based his work on some of the information that had been already published about Vader in publicity materials (movie lobby cards, collectors’ movie programs, and the photo inserts in the Star Wars novelization), plus a Rolling Stone magazine interview with George Lucas. Readers were told that Vader had trained as a Jedi Knight under Obi-Wan Kenobi’s tutelage but had turned to the dark side of the Force, betrayed his master, and killed Luke’s father (who was at this point a separate character from Darth Vader and did not have a name).

The article also had a short and carefully worded explanation of why Vader had to wear his black armor and breath mask. Without going into great detail, May told readers that Darth Vader was a cyborg, that his mechanical suit was akin to a mobile “iron lung,” and that all of this was the result of injuries caused by a titanic lightsaber duel with Obi-Wan near a volcanic pit. At the very end of that fateful duel, Vader fell into the pit and was nearly killed.

James Kahn's novelization of Star Wars: Return of the Jedi expanded on the 1977 reveal that Vader was a cyborg and had been injured in a fateful duel with Obi-Wan Kenobi. © 1983 Del Rey/Ballantine Books and Lucasfilm Ltd. (LFL) 

Six years later, when Ballantine Books published the novelization of Return of the Jedi, James Kahn used May’s material as the foundation of Obi-Wan’s confession to Luke about Anakin’s fall to the dark side. In a far longer bit of exposition than that written by Lawrence Kasdan and George Lucas in the screenplay, Obi-Wan Kenobi tells Luke that when he realized Anakin had fallen under Emperor Palpatine’s thrall, he tried to turn him back to the light. The discussion escalated into an argument, then deteriorated into a full-on clash of lightsabers at the edge of a molten pit. At the climax, Anakin fell into the pit and suffered life-threatening injuries. Disfigured almost beyond recognition, Kenobi’s former apprentice and friend emerged from the pit, not as Anakin Skywalker, but as a hate-filled Darth Vader.

"When I saw what had become of him, I tried to dissuade him, to draw him back from the dark side. We fought…your father fell into a molten pit. When your father clawed his way out of that fiery pool, the change had been burned into him forever—he was Darth Vader, without a trace of Anakin Skywalker. Irredeemably dark. Scarred. Kept alive only by machinery and his own black will." - James Kahn, Star Wars: Return of the Jedi novelization

So, yes, quite a few fans already knew the canonical reason why Vader wore “The Suit” as early as late 1977, and even more of them found out when they read the Return of the Jedi novelization in May of 1983. Star Wars - Episode III: Revenge of the Sith just depicted the more complete version of how Anakin Skywalker became “more machine than man.”