As I've mentioned elsewhere, Darth is initially a seperate character from Anakin in my canon.

The adopted son and second apprentice of Ben, Darth is eventually swayed by Anakin into leaving the Jedi and becoming a Sith. Eventually, the two Sith have a showdown with Ben, but at the last minute Darth has a change of heart and turns on Anakin, causing them both to take the plunge into a river of radioactive waste.

Only one of them survives and manages to crawl out of the river. His features heavily obscured by his burns and the lack of close-ups, it's impossible to tell if this survivor is Anakin or Darth. Palpatine and/or some of his men show up and rescue him and take him back to Coruscant for immediate surgery. Over the next couple of months, this survivor goes through a series of surgeries to graft bionic parts onto his ravaged body and heal what is left of his organic self, and he eventually emerges as an masked-and-armoured cyborg. Even now, though, his name is never revealed.

Leaving the third story, the viewer is left wondering who actually became this armoured cyborg. Entering SW, it would be revealed that the survivor is Darth Vader himself. Obi-Wan would give some information about Anakin to Luke but without revealing that the boy's father turned to the dark side, making it apparent to the viewer that Ben is deliberately hiding this knowledge from Luke. In TESB, Luke would have his first showdown with Vader, leading the viewer to suspect that Vader's going to reveal to Luke that not only did his father fall to the dark side, but he's the one who converted Vader himself. Of course, this is when the plot twist kicks in ...