Quote from Blizzard

The universe is yours with the enigmatic Exodar card back! You can only earn this card back by playing Hearthstone during the month of September.

These card backs replace the art decorating the reverse side of your cards and are a great way to show off your Hearthstone accomplishments. Add the Exodar card back to your collection by hitting Rank 20 in Ranked Play mode. You’ll receive the Exodar card back at the end of September’s Ranked Play Season.

Don’t forget—the further you move up the Ranked Play ladder, the more bonus stars you’ll receive next season. In Ranked Play, you’ll receive bonus stars based on your performance from the previous season —one bonus star per rank you attained. Please note that even if bonus stars from the previous season get you directly to Rank 20 or above, you still have to log in at least once each Ranked Season to qualify for the card back reward.

Board your space vessel of choice and make a flashy entrance into the eighteenth official Ranked Play Season, Exodar’s Exodus, running through the month of September!