In the new Darth Vader comic book Star Wars Darth Vader #10 he sees an alternate ending to Star Wars: Return of the Jedi.
The comic book is by Greg Pak and Raffaele Ienco. It is set between Star Wars: The Empire Strikes Back (Episode V) and Star Wars: Return of the Jedi (Episode VI). In the comic Vader meets up with a space-dwelling psychic squid who gives him the visions. Here is the excerpt below which shows an alternate ending that would have drastically changed Return of the Jedi and the Star Wars universe:
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Source: Star Wars Darth Vader #10
Summary: THE RED HORROR! After passing the test of THE EYE OF WEBBISH BOGG, DARTH VADER has learned the route to the hidden location of THE EMPEROR’S greatest secrets! But in THE RED NEBULA along the way, can Vader survive the onslaught of the greatest predator in the galaxy? Especially if the monster’s most brutal attack transcends the physical? And what horrors await the Emperor if a transformed Vader makes it to his dark door?
(via: Bleeding Cool)