CP3S said:
Did a little searching online and found this quote from the ROTJ novel,
" 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 ... "
I also found that in a pre-Empire Strikes Back interview with Mark Hamill, he mentions that he was told his character's father and Obi-Wan fought Darth Vader over a volcano, and both Luke's father and Vader fall in, with Vader managing to survive.
Good work!
That explains where that all came from. I never read that book or caught the Hamill interview. But the details must have passed from one kid who had, to another, then another in those pre-internet days until it reached my school playground in the North of England lol