User blog comment:Lex wa Kaiho/What is Lumen Histoire?/@comment-198.228.216.47-20140505010536

Lumen Histoire basically translates to History of Light or Historic Light.

Since Light magic is a thing in the fairy tail world maybe this means there exists a "Light" of "fairy" type of Dragon that may have been the Dragon King before Acnologia dethroned him/her. If keeping in the mind the back and forth battle between light and darkness with Acnologia being the ultimate darkness dragon king (since Zeref originally turned him into a dragon) then perhaps Mavis was either a dragon slayer (highly unlikely I know, but maybe.) or (because Greys father is silver) somehow the fairy/light dragon was frozen by Silver and then Mavis stole/took it and kept it underground.

It could also be the essence of the fairy/light dragon. A frozen element would of course be shocking to anybody (including Gildarts)

Welp, theres my small theory.