The start of the October Hearthstone season brings with it a brand new card back for you to work towards unlocking. This month's cosmetic item goes by the name of Unholy Knight, and looks like this:
As always, you don't have to do a great deal of work to add this item to your collection. You simply need to reach Rank 20 on either the Standard or Wild ladder in order to qualify for it, and it'll then be added to your collection at the end of October, along with your usual rewards.
Those rewards are based on the highest rank you manage to achieve during any given month. The higher the climb, the more valuable the rewards you'll receive in terms of crafting dust and golden cards. Note, however, that you can only receive one chest and that's based on the highest rank you achieved on either of the competitive ladders.
Here's the official card back blurb, direct from Blizzard:
"True masters of disease, decay, and undeath, Unholy Death Knights embody the pestilential horror of the Scourge, spreading plague and bringing death wherever they tread! The art on this month’s card back is sick, so make sure your collection contracts the gangrenous green Unholy Knight card back by playing Hearthstone during the month of October."
Don't forget that even if the bonus ladder stars you receive for the previous month's performance put you comfortably in range of the Rank 20 requirement, you'll still have to actually log in at least once during the month to get the latest card back!