It’s about Ben, Luke’s father and Vader when they are young Jedi knights. But Vader kill

It’s about Ben, Luke’s father and Vader when they are young Jedi knights. But Vader kills Luke’s father, then Ben and Vader have a confrontation, just like they have in Star Wars [Episode IV: A New Hope], and Ben almost kills Vader. As a matter of fact, he falls into a volcanic pit and gets fried and is one destroyed being. That’s why he has to wear the suit with a mask, because it’s a breathing mask. It’s like a walking iron lung. His face is all horrible inside.

— George Lucas, August 1977