This kind of pops up in the dialogue of Indiana Jones in the Temple of Doom ("Into the great unknown mystery, I go first, Indy!") after Wu Han is shot, and then it's said again by Palpatine as he's starting to reveal his true identity to Anakin.

"If one is to understand “the great mystery” one must study all it’s aspects, not just the dogmatic narrow view of the Jedi. If you wish to become a complete and wise leader, you must embrace a larger view of the force".

Although Lucas didn't directly write TOD's screenplay (That was his frequent partners William Huyck and Gloria Katz, who also script-doctored part of the Star Wars dialogue) it has been said that along with KOTCS that it has the most Lucas input in it (As Lucas was going through a messy divorce and was in a "dark" mood)


I've heard it's a term that's been used by Lucas in interviews, basically I think it's his basic term for spiruality/religion/bit of an afterlife (although he used "netherworld" in his ROTJ draft and at the end of ROTS as well).