WoW trading card game. Just like in the MMO, the class plays differently than any other -- their website has a series of previews posted that show just how different their DKs are. The class is designed to do crazy damage with two-hand weapons, and there are options to do extra damage by sacrificing allies or equip gear that provides you with solid armor even as you're churning out DPS. There are Death Knight Horde and Alliance allies as well, and each of them has a Death Knight-specific ability -- lots of Shadow damage, exchanging protection for more damage, and even some Frost-tree cold debuffs. And there are quests to go along with the Death Knight class as well, where DKs get extra bonuses for completing and playing certain quests.
Starting on June 16th, you'll be able to pick up 36-card deluxe starter decks for Death Knights at a store near you, and those decks will get inserted into the game just like any other class -- they come with a Hero to play, associated spells and quests, and three "Ghoul token cards" to play as allies. While this is the first time Death Knights will be playable as Heroes, the very first Death Knight cards were actually included in the Wrath of the Lich King Collector's Edition -- a pair of Ally cards named Lord Anton Nightbane (Alliance) and Conqueror Kagon Blackskull (Horde). If you like the TCG but have been looking for a new twist (or a way to bring your Death Knight character into the game), this definitely seems worth a look.
And speaking of the TCG, they are now on Facebook, so if you're a fan, go say hi over there as well. The MMO and TCG crowd still seem to run in different circles (though you'll definitely see the game at BlizzCon), but there are quite a few intersecting points between them.