Darth Mitchell
Super Freak
Fans have dissected the names for years. Here is the official line:
Rolling Stone used to have the interview accessible on their site but the page no longer generates this interview...
I thought I read in the Making of Star Wars that it was from (indeed what Wor-Gar said) Death+ Dark= Darth Invader=Vader
So even George doesn't know what his original idea was