Soldato
I'm not sure (I'm a bit of a noob in WoW), I'm still using the first one you get - Blades of the Fallen Prince on my frost DK, once you put points into all of the traits, another section opens up (on the right hand side in the image from the link below), and once you fill those traits up, the final part in the middle on the right hand side - Concordance of the Legionfall - opens up and then you need to point 50 points into that, all at massively larger amounts of artifact power per point.
http://www.wowhead.com/acquiring-blades-of-the-fallen-prince-frost-death-knight
http://www.wowhead.com/acquiring-blades-of-the-fallen-prince-frost-death-knight