Hearthstone's third official Ranked Play Season kicked off this morning and the accompanying unique card back has since been announced. The rainbow themed card back had been shown off before, but it was unknown what exactly would be required to unlock it. All you'll need to do to obtain it is make rank 20, similar to both the Pandaria and Black Temple themed card motifs.
If you haven't made the grind to rank 20 before, it's not too hard. You just need to win a few games, especially because it's possible to gain a double-star-win-streaks after winning three straight ranked games. Conversely, it's actually impossible to lose stars up to rank 20. If you earned enough stars last season to start below rank 20, you'll still need to log in and play at least one match in order to secure the rainbow card back.