Blizzard's Lead Mission Designer, Dave Kosak, has released a video discussing how Missions will work in Knights of the Frozen Throne.
Missions are the replacement for Adventures, which everyone will be able to attempt when the new set is released. The idea behind them is to be able to explain the lore behind the cards, without forcing people to do the content who are not interested in it.
There is a lot of interesting information in the video, but the key takeaway is that there will be an alternate Paladin Hero, Arthas, awarded to anyone who can defeat the Lich King with all nine classes.
At the end of the video, three more cards are revealed from the upcoming expansion:
Bolvar is an interesting card in that it will be a 3/7 after it loses its own Divine Shield. This means that it will need to get very little value from its ability in order to be very large indeed. It does feel like a build-around card however, and it might be that Divine Shield Paladin is just not as powerful as current archetypes. There are already plenty of ways to build that style of deck, and none are seeing regular play. I am not sure that Bolvar, Fireblood improves the Divine Shield archetype enough to force it to the forefront.
Light's Sorrow has some of the same issues that Bolvar has, although it needs to get very few activations over the course of its lifetime to be a playable weapon. I expect this to be tested heavily in the first few days of the new meta.
Corpsetaker seems like a fun card. It should be easy enough to gain most of the keywords, which will make it a very difficult card to deal with. It should be noted that the keywords are only gained if those words are on the cards remaining in the deck. Depending on how easy it turns out to be to get every keyword onto the card, this could be a reasonable card in slower decks where the Taunt and Lifesteal are relevant.