With TBC Classic content all out and available, the developer recently announced availability for the upcoming Death Knight hero class. The new class is playable by all races and starts at level 55. Making it available right before the new expansion release is the first step in preparing players for the challenges they will face in Northrend.
Because players start at level 55 and can only do one DK per server, Blizzard tries to give players some time to get their characters up to level 70 before WotLK Classic begins.