A new card back has arrived in today's Hearthstone patch which doesn't appear to have a well-defined source!
Mysteries of the Phoenix's in-game description states you can obtain it by:
Solve the mysteries hidden throughout the Barrens.
This card back is related to a new achievement called "???" which has 4 different criteria to finish. Completing these tasks will reward you with 100 experience and the new card back. The card back internally is called "Card_Back_Kael'thas_Sunstrider", which isn't very surprising when you look at the art, and could give us a potential clue to start.
It should be noted that this card back was one of the runner-ups in a competition that was run over in China earlier this year. Congrats to liguang for getting their card back into Hearthstone!