Ancient Order of the Whills

The Whills were a higher order of beings, known to have had Shamans. One of the Shamans was known to have taught Qui-Gon Jinn the secret of the Force ghosts, allowing him to still interact with the living. The Whills kept a collection of stories, called the Journal of the Whills, that chronicled the history of the Galaxy. Whills were thought to be the descendents of the first Jedi.

It is believed that R2-D2, who narrated his stories to a Shaman sometime around 104 ABY for the Journal, was the source of some of the chapters that encompassed the Clone Wars, the Galactic Civil War and the New Republic.

Behind the scenes
In early drafts of A New Hope, the name 'Whills' is equivalent to the Force. It is probably an early incarnation of the concept of midichlorians.

Lucas' quote on this:

"“Originally, I was trying to have the story be told by somebody else (an immortal being known as a Whill); there was somebody watching this whole story and recording it, somebody probably wiser than the mortal players in the actual events. I eventually dropped this idea, and the concepts behind the Whills turned into the Force. But the Whills became part of this massive amount of notes, quotes, background information that I used for the scripts; the stories were actually taken from the Journal of the Whills.” --George Lucas"

Some fans believed that Whill is the name of the unknown race of Yoda, but George Lucas denied this. Also, some fans speculate that the Whills are of spiritual substance, more or less like Wisties, and probably immortal.

Appearances

 * Star Wars Episode III: Revenge of the Sith (mentioned in script, not in final film)
 * Star Wars Episode IV: A New Hope novelization (mentioned in narration)
 * The Last of the Jedi: The Desperate Mission (mentioned only)