"Once you’ve defeated all of the bosses within any given wing for the first time, you’ll unlock the Heroic mode of that wing. Heroic mode bosses are significantly more difficult than their normal versions, and they will prove to be a challenge for even the most veteran Hearthstone players. Defeating all of the bosses in Naxxramas on Heroic mode will unlock a new card back for your collection.
The undead horrors of Curse of Naxxramas: A Hearthstone Adventure are dying to meet you. We hope to see you creeping through the necropolis with your own “All Naxx-cess Pass” soon!"
http://www.hearthpwn.com/news/534-curse-of-naxxramas-wing-entry-details-and-heroic
"Once you’ve defeated all of the bosses within any given wing for the first time, you’ll unlock the Heroic mode of that wing. Heroic mode bosses are significantly more difficult than their normal versions, and they will prove to be a challenge for even the most veteran Hearthstone players. Defeating all of the bosses in Naxxramas on Heroic mode will unlock a new card back for your collection.
The undead horrors of Curse of Naxxramas: A Hearthstone Adventure are dying to meet you. We hope to see you creeping through the necropolis with your own “All Naxx-cess Pass” soon!"
twitter.com/MattDoc
I was under the impression Heroic Mode was how we were going to be able to obtain the Golden Naxx cards. Any word on how we'll do that?
Not yet, still saying they are working on it.