Knights of the Frozen Throne is the second of three expansions Blizzard plans to release this year. This change affords them new, devastating powers. Players don't have to buy the expansion, as the cards can be obtained with in-game gold. Playing through these will earn you free packs from the new set.
The expansion will also include 8 free missions that will pit players against some tough new bosses, all leading up to a final confrontation with the infamous Lich King. Successfully completing each of these Missions will reward players three Knights of the Frozen Throne card packs, as well as a random Legendary Death Knight Hero card. Moreover, Knights of the Frozen Throne brings a new Lifesteal mechanic, already known to many players since it's a standard ability for some creature cards.
Knights of the Frozen Throne will add 135 new cards to the digital trading card game and will mark the first time that two full-size expansions will arrive to Hearthstone in a row.
Place talks about two new hero cards in the video posted below, Malfurion the Pestilent, and Uther of the Ebon Blade. We haven't yet gotten a look at every new card from the expansion, but more than half of them have now been revealed.