It is set in the year 1933, in an alternate history Europe, where magic is real, feudalism persisted, and the Protestant Reformation was crushed by a still politically powerful Catholic Church.

Rex Mundi writer and co-creator Arvid Nelson came up with Rex Mundi while in Paris helping to film a documentary on The Paris Review, a literary magazine founded by Ivy League ex-pats in the 1950s[2][3] It was the first time I had been to Europe and it radically changed my view of the world", Nelson says of the period.

[2] When speaking of his plans for the story, Nelson said that it would remain primarily set in Europe, but the very end will involve a Muslim Spain.

[3] Eric J described the situation in a note to members of the Rex Mundi mailing list announcing his departure from the title: Earlier tonight Arvid informed me that we would not be continuing to work on the book together.

[5]Subsequently, Nelson sent the Image collections to Dark Horse editor Scott Allie.

[6] Rex Mundi devotes space in each issue to a fictitious newspaper, Le Journal de la Liberté, which enables Nelson to embellish events elsewhere in the setting without writing them entirely in his storylines, thus giving readers some useful, but not always essential, background.

The official website for Rex Mundi initially carried a spin-off comic series called Brother Matthew.

In 2006, Jim Uhls was hired to write a script for Johnny Depp to star in and produce.

"[4] An interview with MTV's Splash Page in March 2009 confirmed the film was still progressing and searching for a director.