If you play or have played a DK past level 58, you had the experience of the DK starting area, with the intensely well crafted solo play from doomed soul raised from death by the Lich King to rising power among the armies of the dead to eventual rebellion against the Lich King and his designs for Azeroth. Even if you haven't, if you're level 74 or above I hope you've experienced the Wrathgate event, with its excellent cinematic. (If you have not been to the Wrathgate yet somehow, don't click those links.)
Now, as we digest our BlizzCon diet of news and information from this weekend, we now know that a new expansion is in the works and we, therefore, can anticipate the last act of this expansion when we storm Icecrown Citadel and confront Arthas/Nerzhul in his seat of power. In the very near future, we are going to be fighting the Lich King, and when he eventually is defeated we will have come to an end to this particular story. If you're playing a DK in particular, this is the big moment for you. While we don't know what, exactly, will be happening to Arthas we do know that with his defeat, your DK has done what Darion Mograine set out to do and defeated the very being who raised the Death Knights to their shallow unlife.